Patent Publication Number: US-10317848-B2

Title: Time information display device

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2016-003383, filed Jan. 12, 2016, entitled “Time Information Display Device.” The contents of this application are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a device (time information display device) that has a function of displaying time information, and particularly relates to a time information display device that is installed in a movable body such as a vehicle, and can automatically correct the displayed time information according to the time zone of the current position of the self-device. 
     BACKGROUND 
     A device installed in a movable body such as a vehicle and displaying time should desirably display the correct standard time of the time zone of its current position, even when the self-device moves between time zones with different standard times. As a device for detecting such crossing of a time zone boundary and correcting the displayed time, a conventional time display control device has been known (Japanese Patent Application Publication No. 2010-127808) in which the current position of the self-device is detected on the basis of a GPS (Global Positioning System) signal, for example, whether or not the self-device has crossed a time zone boundary is determined by referring to map information including time zone information, and if the device crosses the time zone boundary, time information held in the self-device is corrected and displayed. Furthermore, to prevent too frequent changes in the displayed time and bothering the driver when driving on a road that frequently crosses a time zone boundary, the device sets an area at a predetermined distance from the time zone boundary, and changes the displayed time only when the vehicle goes beyond the predetermined distance-area toward the destination time zone. 
     However, the position of the self-device located by the GPS signal generally includes an error, and due to this error, it may sometimes be difficult to accurately detect whether or not the vehicle has crossed a time zone boundary. To improve the accuracy in locating the current position of the self-device, in the field of navigation systems, for example, a method has been known in which the position of the device is calculated by use of a GPS signal, and the calculated position is matched with a road position on a map stored in the self-device (so-called map matching). 
     However, generally, map information stored in an onboard device such as an onboard navigation system is rarely updated, and if the map information is updated only at the time of a vehicle inspection, there may be a certain period when the device uses map information that does not include latest information such as newly constructed roads. Accordingly, since onboard devices such as an onboard navigation system do not use latest map information, errors may occur in position location by map matching. 
     As a result, even if the displayed time is kept unchanged within a predetermined distance from a time zone boundary as in the conventional device described above, an error in current position location accuracy may cause frequent changes in the displayed time, depending on the value of the predetermined distance. Additionally, if the area of the predetermined distance is enlarged to avoid such frequent changes in the displayed time, the time may be displayed incorrectly over a needlessly wide area, and usefulness of the time information display device may be degraded. 
     Meanwhile, in recent years, some multifunction mobile terminals such as a smartphone have a GPS function or a map matching function to locate the position of the self-device and perform navigation, or determine the time zone of the current position of the self-device to correct and display time information. Such a mobile terminal communicates with a server, for example, through radio communication or the like, and can always access latest map information. However, since such a mobile terminal cannot include a large high-performance device in view of balance with required lightness, for example, generally, location accuracy of the GPS function tends to be lower than an onboard GPS device. 
     For this reason, in a multifunction mobile terminal, a GPS position calculation error may cause an error in position location by map matching. This may cause fluctuation in displayed time near the time zone boundary, in a manner different from the aforementioned onboard device whose map information may cause an error in position location. 
     SUMMARY 
     In a time information display device installed in a movable body such as a vehicle, it is desirable to effectively prevent fluctuation in displayed time near a time zone boundary that may occur due to accuracy in locating the position of the self-device, for example. 
     An aspect of the present disclosure includes: a communications part that receives, from multiple external devices different from the self-device, information on a time zone of the current position identified by the multiple external devices; a time zone-identification part that identifies a time zone of the current position of the self-device, on the basis of the information received from the multiple external devices; and a display control part that displays time in the identified time zone of the current position on a display part. The time zone-identification part is configured to: determine whether or not the time zone received from the multiple external devices matches a time zone of the current position previously identified by the time zone-identification part; when the time zone received from at least one of the external devices does not match the time zone of the current position previously identified by the time zone-identification part, repeatedly calculate, for a first predetermined time length, an accumulated time in which time zones received from a predetermined number of the external devices match one another; and when the accumulated match time is not shorter than a predetermined time, identify the matched time zone as the time zone of the current position. 
     According to another aspect of the present disclosure, the first predetermined time length is set on the basis of one or multiple factors including: speed of the self-vehicle in which the self-device is installed; an angle between a time zone boundary line nearest to the self-device and a running direction of the self-vehicle; the degree of linearity of the time zone boundary; the degree of linearity of a road on which the self-vehicle runs; and presence or absence of degradation in location accuracy of at least one of the external devices. 
     According to another aspect of the present disclosure, the higher the speed of the self-vehicle, the shorter the first predetermined time length is set. 
     According to another aspect of the present disclosure, the closer the angle between the time zone boundary line and the running direction of the self-vehicle is to a right angle, the shorter the first predetermined time length is set. 
     According to another aspect of the present disclosure, the higher the linearity of the time zone boundary, and/or the higher the linearity of the road on which the self-vehicle runs, the shorter the first predetermined time length is set. 
     According to another aspect of the present disclosure, when location accuracy of at least one of the external devices is deteriorated, the first predetermined time length is set shorter than when the location accuracy is not deteriorated. 
     According to another aspect of the present disclosure, the time zone-identification part is also configured to, when the time zone received from the multiple external devices does not match the time zone of the current position previously identified by the time zone-identification part, for at least one of the external devices, repeatedly calculate each of accumulated times in which the time zone received from the at least one external device matches a first time zone and a second time zone that sandwich a time zone boundary nearest to the self-device, during a latest second predetermined time length period, and also repeatedly calculate a first time ratio that is a ratio of the accumulated time that matches the first time zone to the second predetermined time length, and a second time ratio that is a ratio of the accumulated time that matches the second time zone to the second predetermined time length; and the display control part is also configured to display a time of the first time zone and a time of the second time zone on the display part, in display modes based on the first time ratio and the second time ratio, respectively. 
     According to another aspect of the present disclosure, the display control part is also configured to: display the time of the first time zone on the display part in a larger size for a larger first time ratio; and display the time of the second time zone on the display part in a larger size for a larger second time ratio. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages of the disclosure will become apparent in the following description taken in conjunction with the following drawings. 
         FIG. 1  is a block diagram showing a configuration of a time information display device of a first embodiment of the present disclosure. 
         FIG. 2  is a flowchart showing a procedure of processing of a time determination unit of the time information display device of  FIG. 1 . 
         FIG. 3  is a diagram for describing an example of operation of the time information display device of  FIG. 1 , when crossing a time zone boundary. 
         FIG. 4  is a block diagram showing a configuration of a time information display device of a second embodiment of the present disclosure. 
         FIG. 5  is a diagram for describing the measurement period for calculating first and second time ratios of the time information display device of  FIG. 4 . 
         FIG. 6  is a diagram showing an example of times displayed in first and second time zones on the basis of the first and second time ratios. 
         FIG. 7  is a flowchart showing a procedure of processing of a processing device of the time information display device of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. 
     [First Embodiment] 
     First, a time information display device of a first embodiment of the present disclosure will be described. The time information display device of the embodiment is installed in a vehicle (hereinafter referred to as self-vehicle), and displays time information on an onboard display. The time information display device may be a part of a so-called DA (Display Audio) that includes a display device to display television programs, and provides audio functions such as radio broadcasting and music playback, for example. 
       FIG. 1  is a block diagram showing a configuration of the time information display device of the first embodiment of the present disclosure. A time information display device  100  has a processing device  102  and a storage device  104 . Also, the time information display device  100  includes: an input/output interface (input/output INF)  106  that relays data exchange between a display  150  provided inside the vehicle, and the processing device  102 ; and a communications interface (communications INF)  108  that relays data exchange between the processing device  102 , and an onboard device  152  installed in the vehicle as well as external device  154  such as a smartphone taken into the vehicle and used. 
     The display  150  is a liquid crystal display with a touch panel, for example. 
     The onboard device  152  is an onboard navigation system, for example, and acquires information on the current position of the self-vehicle from an onboard GPS device  156 , for example, determines the time zone of the current position of the self-vehicle by referring to map information included in the onboard device  152 , for example, and transmits information on the determined time zone to the time information display device  100  at predetermined intervals, autonomously or in response to requests from the time information display device  100 . 
     The external device  154  is a multifunction cellular telephone such as a smartphone, or a multifunction mobile terminal (Personal Digital Assistant) such as a portable PC, locates the current position by receiving GPS radio waves or radio waves from cellular base stations, and determines the time zone of the current position of the self-device by referring to map information acquired from a remote server, for example, by radio communication or the like. Also, the external device  154  transmits information on the determined time zone to the time information display device  100  at predetermined intervals, autonomously or in response to requests from the time information display device  100 . 
     The above-mentioned functions of the onboard device  152  and the external device  154  may be implemented by executing an application program on a computer (not shown), included in each of the onboard device  152  and the external device  154 , for example. 
     The communication INF  108  included in the time information display device  100  may be a wire communication-interface compliant with a communication standard such as USB, for example. Note, however, that the communications INF  108  is not limited to this, and may be a wireless communication-interface compliant with a standard such as Bluetooth communication and Wi-Fi communication, or may be a communications interface that performs both of such wireless communication and wire communication mentioned above. 
     The storage device  104  may be a storage device configured of a volatile or nonvolatile memory (e.g., semiconductor memory), or a hard disk, for example. A time determination unit  112  (to be described later) included in the processing device  102  stores, in the storage device  104 , information on the time zone of the current position determined by the time determination unit  112 . 
     Hereinafter, the time zone of the current position determined by the onboard device  152  will be referred to as E-TZ, the time zone of the current position determined by the external device  154  will be referred to as D-TZ, and the time zone of the current position determined by the time determination unit  112  based on E-TZ and D-TZ will be referred to as C-TZ. 
     The processing device  102  is a computer having a processor such as a CPU (Central Processing Unit), a ROM (Read Only Memory) into which a program is written, and a RAM (Random Access Memory) for temporarily storing data, for example, and includes a time zone information-acquisition unit  110 , the time determination unit  112 , a current time-measurement unit  114 , and a display control unit  116 . Here, the communications INF  108  and the time zone information-acquisition unit  110  correspond to a communications part, the time determination unit  112  corresponds to a time zone-identification part, and the display control unit  116  corresponds to a display control part. 
     The above units included in the processing device  102  are implemented by execution of a program by the processing device  102  as the computer, and the computer program can be stored in an arbitrary computer readable storage medium. Instead, or in addition to this, all or some of the above units may each be configured of hardware including one or more electronic circuit parts. 
     The time zone information-acquisition unit  110  acquires, at predetermined intervals, information on the current position-time zone E-TZ determined by the onboard device  152  and information on the current position-time zone D-TZ determined by the external device  154  from the onboard device  152  and the external device  154 , by transmitting request signals to the onboard device  152  and the external device  154  or simply waiting for data transmission from the onboard device  152  and the external device  154 , and sends the information to the time determination unit  112 . 
     The time determination unit  112  determines (or identifies) the current position-time zone C-TZ, on the basis of the current position-time zone E-TZ determined by the onboard device  152  and the current position-time zone D-TZ determined by the external device  154 , which are received through the time zone information-acquisition unit  110 . 
     To be more specific, the time determination unit  112  compares E-TZ and D-TZ acquired at predetermined intervals, and repeatedly measures an accumulated time t match  in which E-TZ and D-TZ match, over a predetermined time length (determination time t det ) period (i.e., in determination time t det  cycles). Then, every time the determination time t det  elapses, the time determination unit calculates a time ratio R 0  (=t match /t det ×100(%)) of the accumulated time t match  to the determination time t det , and determines whether or not the time ratio R 0  exceeds a predetermined threshold R th . If the time ratio R 0  exceeds the predetermined threshold R th , the time determination unit determines that the matched time zone is the current position-time zone C-TZ, and stores information on the current position-time zone C-TZ in the storage device  104 . 
     The current time-measurement unit  114  measures elapsed time with a timer (not shown) included in the processing device  102 , for example, and calculates and identifies the current time in the current position by referring to the time zone (C-TZ) of the current position stored in the storage device  104 . Also, the current time-measurement unit  114  transmits information on the identified current time in the current position to the display control unit  116 . 
     The display control unit  116  displays the current time in the current position received from the current time-measurement unit  114  on the display  150 , through the input/output INF  106 . 
     The time information display device  100  configured in the above manner repeatedly measures the accumulated time t match  in which the current position-time zones E-TZ and D-TZ respectively determined by the onboard device  152  and the external device  154  match to each other, over the predetermined time length t det  period, and if the time ratio R 0  of the accumulated time t match  to the predetermined time t det  exceeds the predetermined threshold R th , determines that the matched time zone is the time zone of the current position. Then, the time information display device displays the time corresponding to the determined time zone on the display  150 . 
     Accordingly, instead of keeping the displayed time unchanged within a predetermined distance from a time zone boundary as in the conventional technique, the time information display device  100  repeatedly calculates the degree of coincidence (specifically, aforementioned time ratio R 0 ) between time zone determination results depending on characteristics (e.g., current position information accuracy and map information accuracy) of the onboard device  152  and the external device  154 , in cycles of the predetermined time length t det , and determines the time zone when the degree of coincidence exceeds a predetermined level (i.e., when time ratio R 0  exceeds threshold R th ). Hence, the time information display device can identify the time zone more promptly and accurately, and prevent fluctuation in displayed time that may occur near a time zone boundary. 
     Next, a procedure of processing of the time determination unit  112  of the time information display device  100  will be described according to the flowchart of  FIG. 2 . The processing starts when the time information display device  100  is powered on, and ends when it is powered off. Note that the display  150 , the onboard device  152 , the external device  154 , and the GPS device  156  start operation before or at the same time as powering on the time information display device  100 . Additionally, in parallel with the processing, the time zone information-acquisition unit  110  acquires, at predetermined intervals, information on the current position-time zone E-TZ determined by the onboard device  152  and information on the current position-time zone D-TZ determined by the external device  154 , from the onboard device  152  and the external device  154 , and transmits them to the time determination unit  112 . 
     When the processing is started, the time determination unit  112  first acquires the latest data on the current position-time zone E-TZ determined by the onboard device  152 , transmitted from the time zone information-acquisition unit  110  (S 100 ). Next, the time determination unit  112  determines whether or not the time zone C-TZ identified as the time zone of the current position is stored in the storage device  104  (S 102 ), and if it is not stored (S 102 , No), stores E-TZ acquired in step S 100  in the storage device  104  as C-TZ (S 104 ), and returns to step S 100  to repeat the processing. 
     Meanwhile, if C-TZ is stored in the storage device  104  (S 102 , Yes), the time determination unit determines whether or not E-TZ acquired from the onboard device  152  in step S 100  and C-TZ stored in the storage device  104  match to each other (S 106 ), and if they match (S 106 , Yes), determines that the time information display device  100  (i.e., self-vehicle including the time information display device  100 ) is not crossing a time zone boundary or is not approaching a time zone boundary, and returns to step S 100  to repeat the processing. 
     Meanwhile, if E-TZ acquired from the onboard device  152  and C-TZ stored in the storage device  104  do not match to each other (S 106 , No), the time determination unit determines that the time information display device  100  (or self-vehicle) has crossed a time zone boundary or is approaching a time zone boundary, and proceeds to step S 108 . In other words, when E-TZ acquired from the onboard device  152  changes from C-TZ stored in the storage device  104 , the time determination unit determines that the time information display device  100  or the self-vehicle has crossed a time zone boundary or is approaching a time zone boundary, and performs the processing in step S 108  and following steps. Instead, the time determination unit may acquire the current time zone (D-TZ) from the external device  154  in step S 100 , and when D-TZ changes from C-TZ stored in the storage device  104 , determine that the time information display device  100  or the self-vehicle has crossed a time zone boundary or is approaching a time zone boundary. Or the time determination unit may acquire both of the current time zone E-TZ of the onboard device  152  and the current time zone D-TZ of the external device  154  in step S 100 , and when E-TZ or D-TZ changes from C-TZ stored in the storage device  104 , determine that the time information display device  100  or the self-vehicle has crossed a time zone boundary or is approaching a time zone boundary. 
     In step S 108 , the time determination unit  112  sets the determination time t det , which is a first predetermined time length, to a predetermined value, and starts measuring elapsed time t elps . The elapsed time t elps  may be measured with a timer (not shown) included in the time information display device  100 , for example. 
     Next, the time determination unit  112  measures or calculates an accumulated time (matched time) t match  in which the time zone (E-TZ) determined by the onboard device  152  and the time zone (D-TZ) determined by the external device  154  match to each other during the determination time t det , based on E-TZ and D-TZ acquired at predetermined intervals through the time zone information-acquisition unit  110  (S 110 ). The matched time t match  may be calculated by the following equation, for example.
 
 t   match   =N×Δt  
 
Here, N is the number of times E-TZ and D-TZ match after the start of measurement in step S 108 , and Δt is the interval at which E-TZ and D-TZ are determined by the onboard device  152  and the external device  154 .
 
     Then, the time determination unit  112  determines whether or not the elapsed time t elps  exceeds the determination time t det  (S 112 ), and if not (S 112 , No), returns to step S 108  to repeat the processing. Meanwhile, if the elapsed time t elps  exceeds the determination time t det  (S 112 , Yes), the time determination unit calculates the time ratio R 0  of the accumulated time t match  to the determination time t det  (S 114 ), and then determines whether or not the time ratio R 0  exceeds the predetermined threshold R th  (S 116 ). And if R 0  does not exceed R th  (S 116 , No), the time determination unit returns to step S 108  to repeat the processing. Thus, the time determination unit repeats calculation of the matched time t match  and the time ratio R 0  periodically by using the determination time t det  as one cycle, until the time ratio R 0  of the matched time t match  of E-TZ and D-TZ within the one cycle exceeds the predetermined threshold R th . 
     Note that when measuring the matched time t match  in step S 110 , if E-TZ and/or D-TZ is changing between a time zone A and a time zone B that sandwich a time zone boundary near the current position, for example, the time determination unit measures each of a matched time t match -A in which both of E-TZ and D-TZ are the time zone A, and a matched time t match -B in which both of E-TZ and D-TZ are the time zone B. Also, similarly, when calculating the time ratio R 0  in step S 114 , the time determination unit calculates each of a time ratio R 0 -A of the matched time t match -A and a time ratio R 0 -B of the matched time t match -B. 
     Then, if any of the time ratios R 0  (i.e., any of aforementioned time ratio R 0 -A and time ratio R 0 -B) exceeds the predetermined threshold R th  (S 116 , Yes), the time determination unit identifies the time zone corresponding to the time ratio R 0  that exceeded the predetermined threshold R th  (e.g., time zone A if time ratio R 0 -A exceeds predetermined threshold R th , or time zone B if time ratio R 0 -B exceeds predetermined threshold R th ) as the current position-time zone C-TZ and stores it in the storage device  104  (S 118 ), and then returns to step S 100  to repeat the processing. 
       FIG. 3  is a diagram for describing an example of operation of the time information display device  100  when a vehicle in which the time information display device  100  is installed crosses a time zone boundary. In  FIG. 3 , a vehicle  300  in which the time information display device  100  is installed runs from the time zone A to the time zone B, on a road  304  that crosses a time zone boundary  302  separating the time zone A from the time zone B. Note that for the sake of clarity,  FIG. 3  shows the vehicle  300  having reached positions F, G, H, J, and K in the drawing, to indicate temporal change in position of the vehicle  300  running on the road  304 . In addition, each of the time ratio R 0 -A of the matched time t match -A when E-TZ and D-TZ are both the time zone A, and the time ratio R 0 -B of the matched time t match -B when E-TZ and D-TZ are both the time zone B at the positions F to K in  FIG. 3  are shown as examples, near the drawings of the vehicle  300  in the respective positions. Note that in this example, the threshold R th  of the time ratio R 0  is 60%, and in position F, the current time zone C-TZ stored in the storage device  104  of the time information display device  100  installed in the vehicle  300  is the time zone A. 
     When the vehicle  300  approaches the time zone boundary  302  from the left in  FIG. 3  on the road  304  and reaches position F, the current position location accuracy of the onboard device  152  (e.g., location accuracy of GPS device  156  connected to onboard device  152 , or accuracy of map information included in onboard device  152 ) causes the current position-time zone E-TZ of the onboard device  152  to indicate the time zone B, which is different from the time zone A indicated by the current time zone C-TZ stored in the storage device  104  (steps S 100  and S 102 , and No in S 106  of  FIG. 2 ). Hence, the time determination unit  112  calculates the time ratio R 0 -A and the time ratio R 0 -B (steps S 108  to S 114  of  FIG. 2 ), and obtains R 0 -A=70% and R 0 -B=20%, as in  FIG. 3 . In this example, Since Rth=60% as mentioned above, and R 0 -A(70%)&gt;R th (60%), the time determination unit  112  identifies the time zone A as the current time zone C-TZ, and stores the time zone A as the current time zone C-TZ in the storage device  104  (steps S 116 , S 118  of  FIG. 2 ). Accordingly, in position F, the current time zone C-TZ stored in the storage device  104  is still the time zone A, and the time displayed on the display  150  by the current time-measurement unit  114  and the display control unit  116  does not change from the standard time of the time zone A. 
     When the vehicle  300  further approaches the time zone boundary  302  and reaches position G, the time determination unit  112  calculates the time ratio R 0 -A and the time ratio R 0 -B in a procedure similar to that described above. Although position G is closer to the time zone boundary  302  than position F, it is still within the time zone A, and therefore the time determination unit obtains numeric values R 0 -A=50%, R 0 -B=30% where R 0 -A&gt;R 0 -B, for example, as in  FIG. 3 . Then, since R th (60%)&gt;R 0 -A(50%)&gt;R 0 -B(30%) (No in step S 114  of  FIG. 2 ), the time determination unit  112  continues calculation of the time ratios R 0 -A, R 0 -B without identifying the current time zone C-TZ. Accordingly, in this position G, too, the current time zone C-TZ stored in the storage device  104  is still the time zone A, and the standard time of the time zone A is still displayed on the display  150 . 
     The vehicle  300  continues to run on the road  304 , and reaches position H on the time zone boundary. The time determination unit  112  calculates the time ratios R 0 -A=30%, R 0 -B=30%, as in  FIG. 3 , and since R th (60%)&gt;R 0 -A(30%)=R 0 -B(30%) (No in step S 114  of  FIG. 2 ), continues calculation of the time ratios R 0 -A, R 0 -B without identifying the current time zone C-TZ. Accordingly, in position H, too, the current time zone C-TZ stored in the storage device  104  does not change from the time zone A, and the standard time of the time zone A is still displayed on the display  150 . 
     Next, the vehicle  300  crosses the time zone boundary  302  and enters the time zone B, and reaches position J. The time determination unit  112  calculates the time ratios R 0 -A=30%, R 0 -B=40%, as in  FIG. 3 , and since R th (60%)&gt;R 0 -B(40%)&gt;R 0 -A(30%) (No in step S 114  of  FIG. 2 ), continues calculation of the time ratios R 0 -A, R 0 -B without identifying the current time zone C-TZ. Accordingly, in position J, too, the current time zone C-TZ stored in the storage device  104  does not change from the time zone A, and the standard time of the time zone A is still displayed on the display  150 . 
     Then, when the vehicle  300  moves away from the time zone boundary  302  and further into the time zone B to position K, the time determination unit  112  calculates the time ratios R 0 -A=10%, R 0 -B=80%, as in  FIG. 3 . Then, since R 0 -B(80%)&gt;R th (60%), the time determination unit  112  identifies the time zone B as the current time zone C-TZ, and stores the time zone B as the current time zone C-TZ in the storage device  104  (steps S 116 , S 118  of  FIG. 2 ). Thus, in position K, the time displayed on the display  150  is switched from the standard time of the time zone A to the standard time of the time zone B. 
     As described above, in the time information display device  100 , the current time zone C-TZ stored in the storage device  104  (i.e., current time zone C-TZ used for displaying time) does not change from the time zone A, in positions G to J near the time zone boundary  302  where R 0 -A and R 0 -B are not larger than R th , and the current time zone C-TZ is only changed to the time zone B in position K where R 0 -B takes a value that exceeds R th . Hence, fluctuation in displayed time that may occur near the time zone boundary  302  can be suppressed, as compared to a conventional configuration where the time zone is determined by using only one of the onboard device  152  and the external device  154  to change the displayed time. 
     Also, in the time information display device  100 , the current time zone C-TZ is changed to the time zone B in position K where R 0 -B takes a value that exceeds R th  (i.e., position of changing current time zone C-TZ is determined based on value of R 0 -B), as described above. Hence, if the value of R 0 -B exceeds R th  in position J that is closer to the time zone boundary  302  depending on the individual difference of the location accuracy (or time zone determination accuracy) of the onboard device  152  and the external device  154 , the current time zone C-TZ is changed to the time zone B in the position J (i.e., earlier, before vehicle  300  reaches position K), and the standard time of the time zone B is displayed on the display  150 . Accordingly, the time information display device  100  can prevent a problem that incorrect time is displayed over a needlessly wide area, so that excellent usefulness of the time information display device can be maintained. 
     Note that although in the embodiment, the determination time t det  as the cycle of measuring the matched time t match  of E-TZ and D-TZ is set to a predetermined time (step S 108 ), the invention is not limited to this. Instead, the determination time t det  may be adaptively shortened or extended relative to the predetermined time, on the basis of one or multiple factors including: speed of the self-vehicle including the time information display device  100 ; an angle between a time zone boundary line and the running direction of the self-vehicle; the degree of linearity of the time zone boundary; the degree of linearity of the road on which the self-vehicle runs; and presence or absence of degradation in location accuracy of the GPS device  156  used by the onboard device  152  and/or the external device  154  (e.g., presence or absence of degradation in location accuracy (e.g., reception quality of GPS radio waves), assumed from geographic features (periphery of cliff, street canyon, surroundings of avenue, for example) of current position indicated by map information). 
     More specifically, the determination time t det  may be set in the following manner. 
     (1) Speed 
     At high speed, the self-vehicle, after passing a time zone boundary, is assumed to move away quickly from the boundary area where time zone identification tends to fluctuate. Hence, the determination time t det  may be set shorter than the predetermined time, by multiplying the predetermined time by a predetermined coefficient K 1  smaller than 1, for example. Note that the value of K 1  may be a continuous value that decreases with increase in speed (e.g., value inversely proportional to speed). 
     (2) Angle Between Time Zone Boundary Line and Running Direction of Self-Vehicle 
     The closer an angle between a time zone boundary line and the running direction of the self-vehicle is to a right angle, the quicker the self-vehicle is assumed to move away from the time zone boundary area where time zone identification tends to fluctuate, after passing the boundary. Hence, the determination time t det  may be set shorter than the predetermined time, by multiplying the predetermined time by a predetermined coefficient K 2  smaller than 1, for example. Note that the value of K 2  may be a continuous value that decreases with decrease in an angle of deviation from a right angle (90 degrees), of the angle between the time zone boundary line and the running direction of the self-vehicle (e.g., value proportional to angle of deviation). 
     (3) Linearity of Time Zone Boundary or Road on which Self-Vehicle Runs 
     When a time zone boundary or the road on which the self-vehicle runs has low linearity and is winding, the vehicle is assumed to stay longer within the boundary area where time zone identification tends to fluctuate. Hence, the determination time t det  may be set longer than the predetermined time, by multiplying the predetermined time by a coefficient K 3  larger than 1, for example, to prevent fluctuation in the displayed time caused by fluctuation in time zone identification. Note that the value of K 3  may be a continuous value that increases with increase in a meander width per predetermined distance along the time zone boundary or the road on which the self-vehicle runs (e.g., value proportional to deviation distance (maximum value of distance deviation in direction perpendicular to approximate straight line of the time zone boundary or road) of the time zone boundary or road from the approximate straight line, within predetermined distance). 
     (4) GPS Accuracy 
     When it is assumed that location accuracy of the GPS device  156  used by the onboard device  152  and/or the external device  154  is deteriorated (e.g., when deterioration of GPS location accuracy is assumed from geographic features (periphery of cliff, street canyon, surroundings of avenue, for example) of current position indicated by map information), the determination time t det  may be set longer than the predetermined time when the deterioration is not assumed, by multiplying the predetermined time by a coefficient K 4  larger than 1, for example, to prevent fluctuation in the displayed time caused by fluctuation in time zone identification. 
     (5) Combination of Above (1) to (4) 
     When at least two of the above (1) to (4) conditions are applicable, the determination time t det  may be set in view of the respective conditions, by successively multiplying the predetermined time by the coefficients K 1  to K 4  set for the respective conditions. 
     Also, although in the embodiment, the time information display device  100  acquires the determination result of the time zone of the current position from two external devices (i.e., onboard device  152  and external device  154 ), the invention is not limited to this. Instead, the determination result of the time zone of the current position may be acquired from three or more external devices. In this case, instead of steps S 100  to S 102  and S 103  of  FIG. 2 , it may be determined that the self-device has crossed a time zone boundary or is approaching the boundary, when the time zone acquired from at least one of the external devices differs from C-TZ stored in the storage device  104 . Additionally, instead of step S 110  of  FIG. 2 , an accumulated time in which time zones received from the predetermined number of external devices match may be measured as the matched time t match . 
     [Second Embodiment] 
     Next, a time information display device of a second embodiment of the present disclosure will be described. In addition to the functions of the time information display device  100  of the first embodiment, when determining that the vehicle has crossed a time zone boundary between a first time zone and a second time zone or is approaching the time zone boundary, the time information display device of the embodiment measures an accumulated time t 1  in which a time zone E-TZ determined by an onboard device  152  is the first time zone, and an accumulated time t 2  in which the time zone E-TZ is the second time zone, for each measurement time t mes , which is the latest second predetermined time length. Then, the information display device displays both of the current time of the first time zone and the current time of the second time zone on a display  150 , in display modes respectively based on a ratio R 1  of the accumulated time t 1  to the measurement time t mes  and a ratio R2 of the accumulated time t 2  to the measurement time t mes . 
     Thus, the time information display device of the second embodiment allows the user to intuitively grasp the likelihood that the current position belongs to the first time zone and that the current position belongs to the second time zone, from the display modes of the two times displayed on the display  150 . 
     Also, the time information display device repeatedly calculates times t 1  and t 2  for the latest measurement time t mes  period. In other words, every time the time zone E-TZ determined by the onboard device  152  is acquired at the predetermined interval, the time information display device repeats calculation of the times t 1  and t 2  as well as R 1  and R 2  for the latest measurement time t mes  period, and updates the modes of time display. Hence, the user can know, in real time, changes in the likelihood that the current position belongs to the first time zone and that the current position belongs to the second time zone. As a result, the user can know from the above-described changes that the self-vehicle is approaching or moving away from a time zone boundary, for example, or that the road is meandering relative to the time zone boundary and therefore the distance to the time zone boundary from the vehicle is varying, for example. 
       FIG. 4  is a diagram showing a configuration of the time information display device of the second embodiment. Note that in  FIG. 4 , the same components as those shown in  FIG. 1  are assigned the same reference numerals as in  FIG. 1 , and the above description of  FIG. 1  will be used. 
     A time information display device  400  of the embodiment has a similar configuration as the time information display device  100  of the first embodiment shown in  FIG. 1 , but is different in that it has a processing device  402  instead of the processing device  102 . In addition, although the processing device  402  has a similar configuration as the processing device  102  shown in  FIG. 1 , it has a time determination unit  412  and a display control unit  420  instead of the time determination unit  112  and the display control unit  116 . Additionally, the processing device  402  also has a first time measurement unit  416  and a second time measurement unit  418 . Here, the time determination unit  412  corresponds to a time zone-identification part, and the display control unit  420  corresponds to a display control part. 
     The above units included in the processing device  402  are implemented by execution of a program by the processing device  402  as a computer, and the computer program can be stored in an arbitrary computer readable storage medium. Instead, or in addition to this, all or some of the above units may each be configured of hardware including one or more electronic circuit parts. 
     As in the case of the time determination unit  112  of the first embodiment, the time determination unit  412  determines a current position-time zone (current time zone) C-TZ, on the basis of the current position-time zone E-TZ determined by the onboard device  152  and a current position-time zone D-TZ determined by an external device  154 , through a time zone information-acquisition unit  110 , and stores information on the time zone C-TZ in a storage device  104 . 
     Moreover, when determining that the vehicle has crossed a time zone boundary between the first time zone and the second time zone or is approaching the time zone boundary, the time determination unit  412  measures the accumulated time t 1  in which the time zone E-TZ determined by the onboard device  152  is the first time zone, and the accumulated time t 2  in which the time zone E-TZ is the second time zone, for each measurement time t mes , which is the latest second predetermined time length. The time determination unit also calculates the ratio (first time ratio) R 1  of the accumulated time t 1  to the measurement time t mes , and the ratio (second time ratio) R 2  of the accumulated time t 2  to the measurement time t mes . Then, the time determination unit  412  stores the information identifying the first time zone and the first time ratio R 1  in the storage device  104  as first time zone information, and stores the information identifying the second time zone and the second time ratio R 2  in the storage device  104  as second time zone information. Note that every time the time zone E-TZ determined by the onboard device  152  is acquired at the predetermined interval from the onboard device  152 , the time determination unit  412  repeats calculation of the accumulated time t 1  and accumulated time t 2  as well as R 1  and R 2 , for the latest measurement time t c , period. 
       FIG. 5  is a diagram for describing the measurement period for calculating the accumulated times t 1 , t 2  as well as the first and second time ratios R 1 , R 2 . In  FIG. 5 , the horizontal axis indicates the flow of time t. Times T 1 , T 2 , . . . T 15 , . . . are spaced apart from one another at constant intervals Δt, and the time zone information-acquisition unit  110  repeatedly acquires the current position-time zone E-TZ from the onboard device  152  at these times. The measurement time t mes  may be set as a multiple of Δt, for example, and is set to t mes =Δt×10 in the example of  FIG. 5 . 
     When the current time reaches time T 10 , for example, a period  1  in  FIG. 5  is regarded as the measurement time t mes  period closest to the current time, and the time determination unit  412  calculates the accumulated times t 1 , t 2  and the time ratios R 1 , R 2  by use of ten E-TZs acquired in time T 1  to T 10  within the period  1 . Assuming that E-TZ acquired in time Tn (n=1, 2, . . . ) continues in period Tn−Δt/2˜Tn+Δt/2, the values are calculated by use of the following equations.
 
 t   1   =N   E-TZ=TZ1 ( T 1: T 10)×Δ t  
 
 t   2   =N   E-TZ=TZ2 ( T 1: T 10)×Δ t  
 
 R 1= t   1   /t   mes ×100(%)
 
 R 2= t   2   /t   mes ×100(%)
 
Here, N E-TZ=TZ1 (T 1 :T 10 ) is the number of E-TZs in which E-TZ=time zone  1  (i.e., the number of times when E-TZ=time zone  1 ), among E-TZs acquired during time T 1  to T 10 , and N E-TZ=TZ2 (T 1 :T 10 ) is the number of E-TZs in which E-TZ=time zone  2  (i.e., the number of times when E-TZ=time zone  2 ), among E-TZs acquired during time T 1  to T 10 .
 
     Then, when the current time reaches time T 11 , a period  2  in  FIG. 5  is regarded as the measurement time t mes  period closest to the current time, and the time determination unit  412  calculates the accumulated times t 1 , t 2  and the time ratios R 1 , R 2  by use of ten E-TZs acquired in time T 2  to T 11  within the period  2 . More specifically, in equation (2), N E-TZ=TZ1 (T 2 :T 11 ), which is the number of E-TZs in which E-TZ=time zone  1  among E-TZs acquired during time T 2  to T 11 , is used instead of N E-T=TZ1 (T 1 :T 10 ), and N E-TZ=TZ2 (T 2 :T 11 ), which is the number of E-TZs in which E-TZ=time zone  2  among E-TZs acquired during time T 2  to T 11 , is used instead of N E-TZ=TZ2 (T 1 :T 10 ) to calculate the accumulated times t 1 , t 2  and the time ratios R 1 , R 2 . 
     Thereafter, similarly, when the current time reaches T 12 , T 13 , . . . , periods  3 ,  4 , . . . shown in  FIG. 5  are respectively regarded as the measurement time t mes  period closest to the current time, and the time determination unit calculates the accumulated times t 1 , t 2  and the time ratios R 1 , R 2  by use of ten E-TZs acquired in the respective periods  3 ,  4 , . . . . 
     Referring back to  FIG. 4 , the first time measurement unit  416  measures elapsed time by use of a timer (not shown) included in the processing device  402 , for example, and refers to the first time zone information stored in the storage device  104  to identify the current time of the first time zone indicated by the first time zone information. Also, the first time measurement unit  416  transmits information on the identified current time in the current position and the first time ratio R 1  included in the first time zone information, to the display control unit  420 . 
     Similarly, the second time measurement unit  418  measures elapsed time by use of a timer (not shown) included in the processing device  402 , for example, and refers to the second time zone information stored in the storage device  104  to identify the current time of the second time zone indicated by the second time zone information. Also, the second time measurement unit  418  transmits information on the identified current time in the current position and the second time ratio R 2  included in the second time zone information, to the display control unit  420 . 
     As in the case of the display control unit  116  of the first embodiment, the display control unit  420  displays the current time in the current position received from a current time-measurement unit  114  on the display  150 , through an input/output INF  106 . 
     The display control unit  420  also refers to the first time zone information and the second time zone information stored in the storage device  104 , and displays the current time of the first time zone and the current time of the second time zone on the display  150 , in display modes based on the first and second time ratios R 1 , R 2 , respectively. 
     For example, the display control unit  420  displays the current time of the first time zone in a larger size for a larger first time ratio R 1 , and displays the current time of the second time zone in a larger size for a larger second time ratio R 2 , on a display screen of the display  150 . More specifically, the display control unit displays the current time of the first time zone within an area S×R 1 /(R 1 +R 2 ), and the current time of the second time zone within an area S×R 2 /(R 1 +R 2 ), relative to a predetermined size area S on the display  150 , for example. 
       FIG. 6  is a diagram showing an example of times displayed in first and second time zones on the basis of the first and second time ratios R 1 , R 2 . Parts (a) to (d) of  FIG. 6  show changes in the times displayed in of the first and second time zones, when the time information display device  400  crosses a time zone boundary from the first time zone to the second time zone. 
     Part (a) of  FIG. 6  shows the displayed time when the time information display device  400  is in the first time zone, and approaches the time zone boundary (more specifically, when it is determined No in S 106  of processing similar to  FIG. 2 ). According to the calculation of this example, the first time ratio R 1  is 80% and the second time ratio R 2  is 20%, for example, and the times are displayed on the display  150 , such that display parts (rectangular parts surrounded by solid line in  FIG. 6 ) of a first time display  600  and a second time display  602  form rectangular parts, whose areas are 80% and 20% of respective rectangular parts (dotted line in  FIG. 6 ) having predetermined areas S. Note that a time  630  based on the current time zone C-TZ identified by the time determination unit  412  is also displayed on the display  150  in parts (a) to (d) of  FIG. 6 . 
     Part (b) of  FIG. 6  shows the displayed time when the time information display device  400  is on the time zone boundary between the first time zone and the second time zone, for example, and the first and second time ratios R 1 , R 2  are both 50%, for example. Here, the times are displayed such that display parts of a first time display  604  and a second time display  606  form rectangular parts, whose areas are both 50% of the respective rectangular parts (dotted line in  FIG. 6 ) having the predetermined areas S. 
     Part (c) of  FIG. 6  shows the displayed time when the time information display device  400  crosses the time zone boundary and enters the second time zone, for example. According to the calculation of this example, the first time ratio R 1  is 20% and the second time ratio R 2  is 80%, for example, and the times are displayed such that display parts of a first time display  608  and a second time display  610  form rectangular parts, whose areas are 20% and 80% of the respective rectangular parts (dotted line in  FIG. 6 ) having the predetermined areas S. 
     Part (d) of  FIG. 6  shows the displayed time when the time information display device  400  moves further away from the time zone boundary than in part (c) of  FIG. 6 , and into the second time zone, for example. According to the calculation of this example, the first time ratio R 1  is 0% and the second time ratio R 2  is 100%, for example, and therefore the first time display is eliminated, while only a second time display  612  is displayed in a rectangular part having the predetermined area S. 
     Note that the inside of the display parts indicated by reference numerals  600  to  610  may be in a different color from the background color, for example. Also, although the numbers showing the time are displayed in a fixed size inside the display parts indicated by the reference numerals  600  to  610  in  FIG. 6 , the invention is not limited to this. Instead, the numbers showing the time may be displayed in different sizes, as long as they stay within the display parts. 
     Moreover, instead of or in addition to displaying the times in display parts of sizes corresponding to the first and second time ratios R 1 , R 2  as in  FIG. 6 , the current time of the first time zone may be displayed in a brightness P×R 1 /(R 1 +R 2 ) and the current time of the second time zone may be displayed in a brightness P×R 2 /(R 1 +R 2 ), relative to a predetermined brightness P of the display screen. Furthermore, the time of the first time zone and the second time zone may be displayed in different display colors based on the first and second time ratios R 1 , R 2  (e.g., varying mixing ratio of red light in white light according to time ratios R 1 , R 2 ), or the frequency at which the displayed times of the first time zone and the second time zone flash may be varied based on the time ratios R 1 , R 2 . 
     Next, a procedure of processing of the time determination unit  412  of the time information display device  400  will be described. 
     In addition to the same processing as in the time determination unit  112  of  FIG. 2 , the time determination unit  412  performs detailed display processing shown in  FIG. 7 , in parallel with the aforementioned processing. Note that the only difference in the processing similar to  FIG. 2  performed by the time determination unit  412  is that the agent performing the steps is the time determination  412  instead of the time determination unit  112 , and therefore the above description of  FIG. 2  will be used. 
     Hereinbelow, the detailed display processing performed by the time determination unit  412  will be described, according to the flowchart shown in  FIG. 7 . This processing starts when it is determined No in S 106  in the same processing as  FIG. 2  performed by the time determination unit  412 , that is, when it is determined that the time information display device  400  (or a vehicle including the time information display device  400  (hereinafter referred to as self-vehicle)) has crossed a time zone boundary or is approaching a time zone boundary. Also, this processing ends when the time determination unit  412  performs S 116  in the same processing as  FIG. 2 , that is, when the time determination unit  412  identifies the current position-time zone C-TZ. 
     When the processing starts, the time determination unit  412  first sets C-TZ stored in the storage device  104  as the first time zone, and a time zone E-TZ most recently received from the onboard device  152  as the second time zone (S 200 ). Next, the time determination unit  412  sets the measurement time t mes  to a predetermined value (S 202 ), and acquires the latest data on the current time zone (E-TZ) determined by the onboard device  152  (S 204 ). 
     Next, the time determination unit  412  calculates the ratio (first time ratio) R 1  of the accumulated time t 1  in which E-TZ is the first time zone, to the measurement time t mes , and the ratio (second time ratio) R 2  of the accumulated time t 2  in which E-TZ is the second time zone, to the measurement time t mes , in the latest measurement time t mes  (S 206 ). 
     Then, the time determination unit  412  stores the first time zone information including information identifying the first time zone and the first time ratio R 1  in the storage device  104 , and stores the second time zone information including information identifying the second time zone and the second time ratio R 2  in the storage device  104  (S 208 ), and then returns to step S 204  to repeat the processing. 
     Note that although in the embodiment, the first and second time ratios are obtained by use of the current position-time zone E-TZ determined by the onboard device  152 , the invention is not limited to this. Instead, the first and second time ratios R 1 , R 2  may be obtained by use of the current position-time zone D-TZ determined by the external device  154 . 
     Additionally, although in the embodiment, the time information display device  400  acquires the determination result of the time zone of the current position from two external devices (i.e., onboard device  152  and external device  154 ), the invention is not limited to this. Instead, the determination result of the time zone of the current position may be acquired from three or more external devices, and the first and second time ratios may be obtained by use of the time zone of the current position determined by any one of the external devices. 
     Instead, the first and second time ratios R 1 , R 2  may be obtained by using t 1  as the accumulated time in which both of the time zone E-TZ determined by the onboard device  152  and the time zone D-TZ determined by the external device  154  are the first time zone, and t 2  as the accumulated time in which both of E-TZ and D-TZ are the second time zone. 
     As has been described, the time information display devices  100  and  400  according to the first and second embodiments of the present disclosure repeatedly acquire, at predetermined intervals, the current position-time zones E-TZ and D-TZ respectively determined by the onboard device  152  and the external device  154  that locate the current position and determine the time zone of the current position, from the onboard device  152  and the external device  154 ; repeatedly measure the accumulated time in which E-TZ and D-TZ match, for a predetermined time length; and when the accumulated time exceeds a predetermined threshold, identify the matched time zone as the time zone of the current position. Then, the time information display devices display the time corresponding to the identified time zone on the display  150 . 
     With this configuration, instead of keeping the displayed time unchanged within a predetermined distance from a time zone boundary as in the conventional technique, the time zone information display devices  100  and  400  of the first and second embodiments collect time zone determination results depending on characteristics (e.g., current position information accuracy and map information accuracy) of the onboard device  152  and the external device  154  for a predetermined time length. Hence, the time information display devices can identify the time zone more promptly and accurately, and prevent fluctuation in displayed time that may occur near a time zone boundary. Although a specific form of embodiment has been described above and illustrated in the accompanying drawings in order to be more clearly understood, the above description is made by way of example and not as limiting the scope of the invention defined by the accompanying claims. The scope of the invention is to be determined by the accompanying claims. Various modifications apparent to one of ordinary skill in the art could be made without departing from the scope of the invention. The accompanying claims cover such modifications.