Calendar mechanism for a timepiece

A calendar mechanism for a timepiece suitable for indicating a piece of information having a period that varies according to at least a first cycle and a second cycle is provided. The mechanism, inter alia, is a system for actuating a lever, in which the actuating system includes a first feeler-spindle intended to come into contact with the first cam and a second feeler-spindle intended to come into contact with the second cam the feeler-spindles being kinematically linked with each other and the actuating system being arranged to move the lever from the inactive position to the active position under the control of each of the cams.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a § 371 national stage entry of International Application No. PCT/EP2016/077398, filed Nov. 11, 2016, which claims priority to Swiss European Patent Application No. 01656/15, filed Nov. 13, 2015, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to the field of horology. It more particularly relates to a calendar mechanism suitable for indicating information having a period varying based on at least a first cycle and a second cycle, such as a perpetual calendar.

BACKGROUND OF THE INVENTION

Document EP 1,351,104 describes a perpetual calendar mechanism: This calendar comprises a maximum month length of 31 days, which varies over a first cycle of 12 months, and which comprises the sequence of 31-28-31-30-31-30-31-31-30-31-30-31 days per month. A second cycle of 4 years for the Julian leap year is superimposed on this first cycle, which adds one additional day to the month of February. The retractable tooth carried by a sliding lever interacts with a 24-hour wheel at the end of the month of February in non-leap years in order to advance the indication of the date from an indication of 28 directly to an indication of 1 of the following month. This tooth is retracted during leap years so that the indication can show February 29, before being advanced to 1, 24 hours later. Nevertheless, this mechanism is complex and fragile.

Document EP 1,818,738 also incorporates a third cycle of 100 years, in order to eliminate February 29th for years divisible by 100, and a fourth cycle of 400 years in order to reestablish it for years divisible by 400. This mechanism therefore makes it possible to display the entire cycle of the Gregorian calendar.

Document EP 0,606,576 describes a Muslim calendar mechanism. The Muslim calendar is based on the lunar cycle, and comprises a first cycle of 12 months, the odd months comprising 30 days, and the even months comprising 29 days. In order to compensate for the difference between this cycle and the full Muslim year, the twelfth month of certain years has 30 days instead of 29, according to a second 30-year cycle. Several variants of this second cycle exist, but one variant commonly used defines the years in which the twelfth month comprises 30 days instead of 29 as follows: 2nd, 5th, 7th, 10th, 13th, 15th, 18th, 21st, 24th, 26thand 29th.

These calendar mechanisms of the prior art are relatively complex, and require a large amount of space in the movement.

The aim of the present invention is to propose such a calendar mechanism that is simple, compact and reliable.

BRIEF DESCRIPTION OF THE INVENTION

More specifically, the invention relates to a calendar mechanism for a timepiece, said calendar mechanism being adapted to indicate information having a period varying according to at least a first cycle and a second cycle. “First cycle” for example refers to a month cycle whereof the number of days of each month is invariable from one year to the next, for example from the first to the eleventh month of the Islamic calendar, or the months of January and March to December of the Western calendar (Julian or Gregorian). “Second cycle” for example refers to a cycle with a length different from the first, for example the 30-year cycle that determines the number of days in the twelfth month of the Islamic calendar, the four-year cycle of the Julian calendar or the 400-year cycle of the Gregorian calendar, which determines the number of days in the month of February. These two cycles are superimposed in order to provide the desired indications over a full cycle.

This mechanism comprises a months wheel comprising a gear having a number of fixed teeth, said number being chosen based on the maximum period of the information to be displayed. Normally, this number of teeth is the same as the maximum number of days in a month, or its whole multiple.

The wheel further comprises a lever arranged to move between an inactive position and an active position and vice versa, this lever being provided with at least one tooth which is retractable relative to the perimeter of the wheel. This at least one tooth can be a traditional tooth extending in the plane of the lever, or a tooth extending perpendicular to this plane in the form of a contrate tooth, pin, lug or the like, which are typically all considered “teeth” when they perform the same function.

The mechanism further comprises a driving wheel comprising a first driving organ arranged to interact with said fixed teeth and a second driving organ angularly offset relative to the first driving organ and arranged to interact with said retractable tooth when said lever is in its active position. In this position, the retractable teeth are positioned so as to be able to cooperate with the driving wheel.

Incorporated into said mechanism are a first cam whose shape represents the variations of said period according to the first cycle, and a second cam whose shape represents the variations of said period according to the second cycle, as well as an indexing system kinematically connected with said wheel, with said first cam and with said second cam and suitable for indexing each of said cams as a function of said cycles.

An actuating system of said lever is also incorporated into said mechanism. This actuating system comprises a first cam feeler-spindle intended to come into contact with the first cam and a second cam feeler-spindle intended to come into contact with the second cam, said feeler-spindles being kinematically connected to each other and the actuating system being arranged to cause said lever to move from its inactive position to its active position under the control of each of said cams.

Consequently, these two kinematically-connected feeler-spindles allow the lever to be actuated either based on information carried by the first cam, or based on information carried by the second cam, such that it is commanded based on the superposition of the two cycles in order to display the desired complete cycle, and consequently the months wheel is advanced by the appropriate number of steps at the end of each month.

Advantageously, said first cam and said second cam are each coaxial to said gear and carried by the latter. This results in a particularly compact construction. Alternatively, these two cams could also not be coaxial to said gear.

Each of said cams can be situated on the same side of said gear, or one on a first side of said gear, and the other on the opposite side.

In this second case, the second feeler-spindle advantageously extends through an opening formed in said gear in order to be able to feel the cam that is situated on the side opposite the first feeler-spindle.

Advantageously, the actuating system comprises a first lever provided with said first feeler-spindle and also provided with a stop intended to come into contact with said lever in order to cause it to go to its active position under the control of said first cam.

In one variant, not only the first feeler-spindle, but also the second feeler-spindle is carried by the first lever, which creates a particularly compact arrangement. Alternatively, the actuating system can comprise a second lever kinematically connected with said first lever, said second lever bearing said second feeler-spindle.

In one variant, the actuating system comprises an additional actuating wheel kinematically connected, directly or indirectly, with said first lever. This additional actuating wheel is arranged to cause said lever to move toward its active position under the control of said second cam.

Advantageously, the additional actuating wheel is pivoted relative to said gear and comprises an additional actuating cam intended to press against said lever under the control of said second cam.

In one variant, said first cam has a shape representative of the number of days in a month varying according to a first cycle, and said second cam has a shape representative of the number of days in at least one particular month that varies according to a second cycle, and in which said first cam comprises a notch representative of at least one month whose number of days is determined according to said second cycle. In one particular alternative in which the first cycle comprises twelve months and the second cycle comprises thirty years, the shape of said first cam represents the number of days from the first to the eleventh month varying according to the first twelve-month cycle, the notch corresponding to the twelfth month, and the shape of said second cam corresponds to the number of days of the twelfth month varying over the second thirty-year cycle. The notch allows the first cam not to influence the position of the first lever for the month where the number of days is determined by the second cam.

Alternatively, the first cycle comprises four years, the shape of said first cam representing the number of days in February varying according to the first four-year cycle (i.e., 28-28-28, then 28 or 29 (depending on the second cycle)), the notch corresponding to the month of February, which may or may not be a leap year for example depending on the Julian or Gregorian calendar. In this alternative, the second cycle comprises 100 years or 400 years, the shape of said at least one cam representing the number of days in February for years divisible by four, which may or may not be leap years depending on the Gregorian or Julian cycle.

Advantageously, the mechanism comprises a second additional cam superimposed on said second cam, the combination of the second cam and the additional second cam defining a 400-year cycle, particularly according to the Gregorian calendar. This makes it possible to avoid using a single cam with 400 sectors, and to use to simpler, superimposed cams. In order to read these cams, the second feeler-spindle can be arranged to feel the second cam and the additional second cam in parallel, i.e., simultaneously.

Advantageously, said lever carries at least two retractable teeth having the same separation as two fixed teeth of the gear. The mechanism is therefore reversible, and maintains its indexing independently of the rotation direction of the driving wheel. Consequently, even when a correction is made in the direction opposite the normal operating direction, correct indexing is therefore provided.

Advantageously, a device for displaying the day of the week can be provided, which is preferably associated with the driving wheel in order to indicate the day of the week simply.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-3schematically illustrate one embodiment of a calendar mechanism1according to the invention, in the form of a Muslim calendar. In order to illustrate the interaction between the various components and to provide a better appreciation of depth in the figures, different types of cross-hatching have been used. Furthermore,FIG. 4illustrates the operating principle in the form of a schematic.

The mechanism1comprises a months wheel3, which comprises a gear4bearing fixed teeth5relative to the wheel4. The number of fixed teeth5is chosen based on the maximum number of days to be displayed, in particular thirty in the illustrated case. A whole multiple of this number is also possible. The positioning of the months wheel3can be provided traditionally using a jumper (not illustrated). The gear4is traditionally associated with one (or several) display organ(s) (not illustrated) that indicate(s) the date.

The months wheel3is driven by a driving wheel7, which comprises a first driving organ7aprovided with four driving teeth or fingers separated by spaces, these latter being configured in order to cooperate with the fixed teeth5. The driving wheel7is in turn arranged to cooperate with the fixed teeth5. The driving wheel7in turn is arranged to be driven by a base movement (not illustrated) at a rate of one quarter-revolution per 24 hours, typically around midnight. The number of driving teeth or fingers can be chosen based on the horologist's needs, and considered generically, if n is the number of driving teeth or fingers, the driving wheel7performs 1/n revolutions per day.

The months wheel3also comprises a lever9, pivoted on the gear4at a pivot point9a, which carries retractable teeth11situated in a plane different from that of the fixed teeth5and secured with the lever9. In the view ofFIG. 1, the lever9and the retractable teeth11are in their—inactive—retracted position, while they are in their—active—deployed position inFIGS. 2 and 3. A return spring (not shown) acts to keep the lever9in its inactive position (FIG. 1). In the active position of the lever9, the retractable teeth11can cooperate with a second driving organ7bcomprised by the driving wheel7. The retractable teeth11are therefore retractable relative to the perimeter of the months wheel3.

This second driving organ7bhas a shape substantially similar to the first driving organ7a, but is angularly offset by ⅛ revolution relative to the latter and is situated in a plane allowing it to cooperate with the retractable teeth11.

Consequently, if the retractable teeth11are in their retracted position (FIG. 1), the second driving organ7bhas no effect, and for each quarter-revolution of the driving wheel7, the months wheel3is pivoted by one step, namely by one tooth in the illustrated case.

If the retractable teeth11are in their active position (FIGS. 2 and 3), a quarter-revolution of the driving wheel7pivots the months wheel3by one step by means of the interaction between the first driving organ7aand the fixed teeth5, and an additional step by means of the interaction between the second driving organ7band the retractable teeth11.

If the retractable teeth11only comprise one tooth11, the calendar mechanism1only works in a single rotation direction. However, in the illustrated case where two retractable teeth11are present and each superimposed on a pair of fixed teeth of the gear4(or are offset by one pair of fixed teeth of the gear4), with a separation similar or identical to the separation between two fixed teeth5of the gear4, the calendar mechanism1works reversibly, i.e., the number of steps performed by the months wheel3after driving in either direction (for example after a manual correction of the date in the direction opposite the typical rotation direction of the months wheel3) remains correct, and the mechanism always remains correctly indexed relative to the displayed date by means of indicator organs associated therewith (not illustrated). If the gear4comprises a number of fixed teeth5that is a multiple of the maximum number of days in a month, the number of retractable teeth can be multiplied by this multiple.

The position of the lever9is controlled by means of an actuating system13based on the position of a first cam15and a second cam17. The first cam15is situated between the pivot point9aof the lever9and the rotation axis of the wheel3, and is arranged to pivot relative to the gear4. The shape of the first cam represents the first 12-month cycle, and therefore has larger radius parts15arepresenting the odd months with 29 days, and smaller radius parts15brepresenting the even months with 30 days. Given that the number of days of the twelfth month depends on the year, this month is represented by a notch15cwhose operation will appear more clearly hereinafter.

The second cam17is situated between the pivot point9aof the lever9and the periphery of the gear4, and is also arranged to pivot relative to the gear4. The shape of the second cam17represents the second 30-year cycle, which determines the number of days of the twelfth month. The shape of the cam17extends inward, and comprises larger radius parts17a, which represent the twelfth months with 29 days, and smaller radius parts17b, which represent the twelfth months with 30 days.

This type of cam is often called “programme cam”, because their shape determines the number of days indicated in a month, and therefore serves to “programme” the sequence indicated by the mechanism1.

The cams15,17are supported and rotated relative to the gear4using an indexing system19, which is shown schematically inFIGS. 1 to 3, and ensures that the angular position of each cam15,17relative to the gear4is correctly indexed for the indicated date. As shown by the arrows, the indexing system19is driven by the gear4, and drives the two cams15,17by means of an appropriate transmission system, such as gears, a Maltese cross, star or any other appropriate system. It goes without saying that this transmission system can control the two cams15,17individually, or can control the second cam17relative to the first15. The details of the indexing system are not part of the invention in itself, and will not be described in more detail.

The actuating system13comprises a first lever21bearing a feeler-spindle21a, the first lever being pivoted on the gear4at a pivot point21bunder the effect of a spring (not shown). The first lever21also bears a stop formed by a stud21carranged to press against a flank of the lever9in order to bring the latter into its active position (FIG. 2). However, said first lever21could also work directly with said lever9without using a stud. Likewise, the lever9could include a stud and the first lever21could not have one. It is also possible for the lever9and the first lever21each to have a stud.

The first feeler-spindle21afollows the first cam15such that, when the first feeler-spindle21ais in contact with a smaller radius portion15bof the latter (FIG. 1), the stud21cdoes not bring the lever9into its active position, and the retractable teeth11are in their retracted position, the second driving organ7bcannot interact with the retractable teeth11, and a 30-day cycle will therefore be displayed.

However, when the first feeler-spindle21ais in contact with a larger radius portion15aof the first cam (FIG. 2), the stud21cbrings the lever9and the retractable teeth11into their active positions. Consequently, the second driving organ7bcan interact with the retractable teeth11, and a 29-day cycle will therefore be displayed because the gear4will advance at a rate of two steps at the end of the month.

During the twelfth month, the first feeler-spindle21ais in the notch15c, and consequently the stud21cis situated away from the lever9. The length of the twelfth month is determined not by the first cam15, by the second cam17.

In order to feel the second cam17, a second lever23is also pivoted on the gear4at a pivot point23b, and is kinematically connected with the first lever21using complementary toothed segments21d,23dcarried by each lever21,23. The second lever23comprises a second feeler-spindle23a, which is intended to come into contact with the second cam17, at least during the twelfth month. If the twelfth month has 30 days, the second feeler-spindle23ais in contact with a smaller radius part of the second cam17, and the actuating device13adopts the configuration illustrated inFIG. 1, with the exception of the fact that the first feeler-spindle21ais across from the notch15c.

If the twelfth month comprises 29 days, the second feeler-spindle23ais across from a larger radius part17aof the second cam17, as illustrated inFIG. 3. The first feeler-spindle21abeing across from the notch15c, it can therefore pivot further toward the rotation axis of the first cam15than in the other positions of the first cam15. The second feeler-spindle23ais therefore free to come into contact with, and be placed against, the larger radius part17aof the second cam17, which causes the second lever23to pivot clockwise (relative to the view ofFIGS. 1 to 3). This rotation is transmitted to an additional actuating wheel25pivoted on the gear4and made up of an additional actuating cam25aarranged to command the lever9, as well as a gear wheel25bthat is kinematically connected with the second feeler spindle23aby means of a transmission lever24. In the illustrated case, the transmission lever24also comprises two toothed segments, or racks, meshing on either side with said toothed wheel25band with a complementary toothed sector comprised by the second lever25.

It goes without saying that any type of kinematic link between the levers21,23,24and the additional actuating wheel25is possible, for example studs cooperating with grooves, a belt or the like.

Indeed, the mechanism represents a globally binary logic system, which can be shown by the following transfer table:

It is clear that the cam1has “priority” in this logic, and that it is only when the first feeler-spindle21ais across from the notch15cthat the cam2can influence the position of the lever9.

In order to avoid conflicts between the various components of the mechanism1, they are located in appropriate planes so that they can pass above one another as needed. For example, inFIG. 1, the transmission lever24is overlapped by the first cam15, which is therefore located in another plane. The same situation also exists with the second cam17, which is overlapped by the transmission lever24inFIG. 3.

FIG. 4schematically illustrates the operating principle of the mechanism1according to the invention. The driving wheel7drives the gear4around midnight as well as driving, if applicable, the retractable teeth11. The retractable teeth11are in their active position and are therefore driven by the second driving organ7b, the force exerted on the lever driving the gear4by one additional step.

The gear4being kinematically connected with the indexing system19, each modification of the angular position of the months wheel3modifies the state of the indexing system, which modifies the position of the first cam15and the second cam17so that they are indexed correctly relative to the gear4for the displayed date. These cams15,17are felt by the first and second feeler-spindles21a,23a, respectively, which therefore determine the position of the lever9.

FIGS. 5 to 8illustrate a second embodiment of a Muslim calendar mechanism1that applies the same principle as that ofFIGS. 1 to 3. These figures illustrate the mechanism1, seen from both sides, in particular the upper side (FIGS. 5 and 6) and lower side (FIGS. 7 and 8), and in perspective (FIGS. 5 and 7) and in plan (FIGS. 6 and 8) views, during a 30-day month. In these figures, the driving wheel7has not been shown, but may be identical to that illustrated inFIGS. 1 to 3or have any other appropriate form.

This embodiment primarily differs from the first in the positioning of the second cam17and the components of the actuating system13.

As visible inFIGS. 5 and 6, the arrangement of the lever9, the first cam15, the first feeler-spindle21aand the stud21cis substantially unchanged.

However, the first lever21extends in a curve so that its rack21dmeshes directly with the toothed sector25bof the additional actuating wheel25. Consequently, the intermediate levers23and24of the first embodiment have been deleted.

Nevertheless, it goes without saying that a different kinematic link between the first lever21and the additional actuating wheel is also possible.

The second cam17is on the opposite side of the gear4, therefore on the face other than that of the first cam15, and is situated centrally, extending outward. The second feeler-spindle23ais a pin secured to the first lever21, which extends through an opening4aformed in the gear4so as to be able to feel the second cam17.

Notwithstanding the structural changes, the operating principle of the mechanism remains unchanged.

FIGS. 9 to 11illustrate the same operating principle as described above, but applied to the handling of the month of February of the Gregorian calendar. This system being more complicated than those described above, certain assembly elements (staffs, pins, etc.) have not been illustrated. Furthermore, the illustration is highly schematic in order to more clearly show its operation, and the elements dealing with the jumps for days 31 and 30 of the month have been shown separately from those dealing with the jump for day 29 of the month, in different figures.

FIG. 9illustrates the elements providing the jump for the 31stday of the month during months with 30 days as well as each month of February, and the jump for the 30th day of the month at the end of each February. These jumps follow a single 12-month cycle, and consequently do not apply the principle of superimposed cycles according to the invention, but are illustrated here in the interest of completeness.

In this embodiment, the gear4of the months wheel1comprises 31 teeth (or alternatively a whole multiple of 31 teeth) that cooperates with a first driving organ7aof the driving wheel7, and during its standard operation, rotates clockwise as seen inFIG. 9at a rate of one revolution per month. The first additional lever27is pivoted on the gear4(its axis having been removed from the figure), and has two teeth arranged similarly to those of the lever9mentioned above and which are positioned so as to be able to interact with an additional driving organ7cof the driving wheel7, situated in a plane other than that of the first driving organ. A second additional lever29is also pivoted on the gear4(its axis also having been removed from the figure), and also has two teeth offset by one step in the upstream direction relative to those of the first additional lever27, and which are positioned in order to be able to interact with still another additional driving organ7dsituated in still another different plane on the driving wheel. These additional levers27,29operate in a manner similar to the lever9, except that they are each controlled by a single cam. Furthermore, in the illustrated embodiment, the driving organs are provided in pairs, offset by 180°. The driving wheel therefore performs a half-revolution per day. Other arrangements are also possible, for example 1, 3 or 4 driving organs per level.

The actuation of these levers being carried out according to a single 12-month cycle, the wheel3comprises a first additional cam31, which is followed by a feeler-spindle27aof the first additional lever27in order to bring this lever into its active position at the end of months with 28, 29 or 30 days. This additional cam31therefore has five protrusions corresponding to the months with fewer than 31 days. A second additional cam33, secured in rotation with the first additional cam31, is also followed by a feeler-spindle29aof the second additional lever28in order to bring this lever into its active position each February, and to that end this additional cam31has a protrusion corresponding to the month of February.

The indexing system19provides the angular relationship between the additional cams31,33and the gear4, while providing a rotation speed ratio of 11/12 or 13/12 of the additional cams31,33relative to the gear4, in the desired direction of relative rotation between these components. However, any other speed ratio between the gear4and the cam31as well as between the gear4and the cam33can be considered, as long as the teeth of the levers27and29are positioned appropriately during their passage in front of the driving wheel7.

FIGS. 10 and 11illustrate the elements that relate to the month of February. The illustrated arrangement shows a modification of the system ofFIGS. 5 to 8, and consequently, only the differences with respect to the embodiment of the latter figures will be described in detail here, the elements having the same reference signs reprising the same functions, mutatis mutandis.

The first cam15, the lever9(the rotation axis9aof which has not been shown), and the components of the actuating system13visible inFIG. 10are superimposed on the additional levers27,29and the additional cams31,33such that the teeth11of the lever9can interact with the second driving organ7bof the driving wheel7.

The first cam15in fact comprises 48 sectors representing the first cycle of 4 years/48 months, most of these sectors of which have the same, smaller, radius15b(months other than February). The three larger radius parts15aare bosses representing the months of February which are always non-leap years and which bring the lever9into its active position in order to jump over the 29th of the month, and a notch15crepresents those of a year that may optionally be a leap year (and therefore comprise 29 days) according to the Gregorian calendar (i.e., each year divisible by 4). The indexing system makes sure that the first cam15performs a quarter-revolution per year relative to the gear4.

In the same way as for the embodiment ofFIGS. 5 to 8, this notch delegates the control of the lever9to at least a second cam17, located on the opposite side of the gear4. In this case, the mechanism comprises not only a second cam17, but also an additional second cam18coaxial to the second. These two cams are felt in parallel by the second feeler-spindle23a. In the illustrated embodiment, the second cam17comprises twenty sectors, three of which have notches, and the second additional cam18comprises five sectors, one of which has a notch. In effect, the combination of these notches provides the information on whether the year in question is divisible by 100 and not by 400.

Looking atFIG. 11, one sees the second cam17, which for example performs one revolution in 400 years relative to the gear4, and the second additional cam18, which performs one revolution in 20 years, also relative to the same gear. Alternatively, the second cam17can for example also perform one revolution in 80 years, and the additional second cam18can perform one revolution in 400 years. A single second cam17having 400 sectors is in fact very difficult to produce in light of the size of such a mechanism, which is why it has been chosen to use the “AND” logic combination of two cams17,18. This combined cycle of 400 years is therefore the second cycle within the meaning of the invention.

Consequently, if a notch of the second cam17and a notch of the additional second cam18are located below the second feeler-spindle23a(which is the case for each year divisible by 100, but not by 400), the latter falls into the combined notch (as long as the feeler-spindle21a, which is located on the opposite side of the gear4, is above the notch15c), and the first lever21pivots in the counterclockwise direction according to the view ofFIG. 10. Its rack21dtherefore controls the additional actuating cam25ato bring the retractable teeth11into their active position, and the 29th day of the month is thus jumped over for the years 1700, 1800, 1900, but not for 1600, 2000, etc. For any other month of February, the year is a leap year and at least one of the second cam17and the additional second cam18prevents the first lever21from rotating and the retractable teeth11remain in their inactive position. Consequently, the 29th day of the month is not jumped over and is therefore displayed. Such a configuration is illustrated inFIG. 11.

This system is also completely reversible and maintains its indexing during a backward correction by means of the driving wheel.

However, the illustrated embodiment is not limiting, and depending on the arrangement, the sizing and the nature of the driving means of the cams17,18, any other appropriate speed ratio between the latter can be considered and is covered by the present invention, as long as the teeth11of the lever9are positioned appropriately during their passage in front of the driving wheel7. The important point is that, during the month of February in leap years (therefore when the feeler-spindle21ahas the possibility of falling into the notch15c) and when the teeth11are close to the driving wheel7, the cams17and18are positioned correctly.

In all of the embodiments, it is also possible to provide a device for displaying any day of the week, which may be associated with the driving wheel7.

Although the invention has been described in reference to several specific embodiments, changes are possible without going beyond the scope of the invention, as defined by the claims. In particular, it should be noted that the same principle may also be applied to a Gregorian, annual or Julian Western, Chinese, Hebrew or similar calendar.