Abstract:
A perpetual calendar for displaying a monthly calendar by aligning the desired monthly indicia with an appropriate yearly indicia. The calendar is based on a repeating five element pattern representing a four year repeating pattern, which, however, is modified for non-leap year centuries.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of calendars and, more particularly, to perpetual calendars. 
     BACKGROUND OF THE INVENTION 
     Perpetual calendars are calendars that can be manipulated to display various different periods of time, such as weeks or months. Perpetual calendars are based upon the known yearly cycle of time as quantified by the months and days. 
     Many perpetual calendars function by aligning a day of the week, e.g., Monday, Tuesday, with the day on which January 1 st  of a given year falls. Once the day of the week on which January 1 st  falls in a given year is known, the dates and corresponding days of the weeks within that year are known based on the known number of days within a week and the dates in each month. 
     Year to year changes on the day of the week on which January 1 st  falls are taken into account based on the fixed number of days in a week and the known number of days in a year. While all weeks have seven days, the number of days in a year varies. 
     All years have 365 days, unless the year is a leap year, which has 366 days. A leap year is any year divisible by 4, except where the year is a century, e.g. 2000, 2100, which is only a leap year if also divisible by 400. Thus, the century 2000 was a leap year but the century 2100 will not be. The known number of days in a year combined with a fixed week of seven days mandates that January 1 st  of a year following a 365 day year begin on the next day of the week from which that year began. For example, if January 1 st  of a 365-day year was on a Monday, the January 1 st  of the following year will be on a Tuesday. In the special case where a year follows a leap year, the January 1 st  of the following year is not one day later but two, to account for the extra day in the 366-day year. For example, if January 1 st  of a 366-day year was on a Monday, January 1 st  of the following year will be on a Wednesday. The day of which January 1 st  falls in preceding years may be similarly obtained. 
     Over the years there have been many structures for perpetual calendars. Many of the calendars, however, do not simultaneously display the days, dates, months and year. Most display only a month with the days and dates therein. In addition, changing the relationship in the calendar to reflect for example months in a different year, particularly a leap year, is complex. Generally, most perpetual calendars make the assumption that a viewer of the calendar is only interested in the current month. 
     Based on the above, it is an object of the present invention to create a perpetual calendar that is more readily adaptable to changing the relationships depicted thereon. 
     It is another object of the present invention to create a perpetual calendar that more easily accommodates leap years. 
     It is still yet another object of the present invention to create a perpetual calendar that displays the entire relationship between the dates, days, months and years. 
     SUMMARY OF THE INVENTION 
     The present invention in one aspect is a perpetual calendar having a body with an outer surface. The outer surface is divided into seven segments, the number of segments corresponding to the number of days in a week. Date indicia for the longest month in a year, 31 indicia in all, are successively positioned on the outer surface in the seven segments. At least five-year indicia, representing a repeating pattern based on four years, are also positioned on the outer surface in each segment. The year indicia positioned in any one segment are based on the date indicia therein. 
     A cap is positioned relative the body and has an outer surface. Positioned on the outer surface of the cap are day indicia and month indicia. The day indicia and month indicia are positioned in a fixed relationship and define seven sections. The seven sections are consistent with the seven segments on the body such that a section aligns with a segment. The cap is positionable about the body permitting the seven sections to align with the seven segments to display a one month calendar for each aligned month and year. Preferably, the cap does not interfere with the viewing of the indicia of at least one month and corresponding year on the body, thus permitting the day, date, month and year to be simultaneously viewed. 
     The indicia within any segment or section can be arranged as desired therein. In addition, additional month ending indicia, indicia to indicate the last day of month, can be added to the segment having the day indicia that indicates the last day of a month. For example, “Apr” can be added in the segment having day indicia  30  to indicate that April has 30 days. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded side perspective view of one embodiment of a perpetual calendar according to the present invention. 
     FIG. 2 is a table illustrating the placement of date indicia, year indicia and month ending indicia within the segments of the cylindrical body of the perpetual calendar depicted in FIG. 1 
     FIG. 3 is a table illustrating the placement of day indicia and month indicia within the sections of the rotating cap of the perpetual calendar depicted in FIG.  1 . 
     FIG. 4 is an assembly view of the perpetual calendar of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 1, a perpetual calendar, generally denoted by the reference number  10 , includes a cylindrical body  12  and a rotating cap  14 . Referring to FIGS. 1 and 2, the cylindrical body  12  has an outer surface  16  which is divided into seven segments  18 . Each segment  18  is divided into a first portion  20  and a second portion  22 , the demarcation of which is denoted by a dotted line. 
     On the cylindrical body  12  within each segment  18  in the first portion  20 , date indicia  24 , in this case numbers, are sequentially positioned. As used herein sequentially positioned means positioning an indicia within a segment with the next sequential indicia positioned in an adjacent segment and so forth. 
     Within each segment  18  in the second portion  22  is at least one year indicia  26  that corresponds to the date indicia  24  already positioned within the segment  18 . As this is a perpetual calendar, there is a fixed relationship between the date indicia  24  and the year indicia  26 . This fixed relationship is based upon the known number of days in the year, 365 or 366, and the fixed pattern of days, weeks and months that define a year. Similarly. the perpetual calendar  10  includes a fixed relationship between the day indicia  36  and month indicia  38  that corresponds with the fixed relationship between the date indicia  24  and year indicia  26 . FIG. 2 provides a layout of date indicia  24  and year indicia  26 . FIG. 3 provides a layout of day indicia  36  and month indicia  38 . While FIG. 2 is complete as to the layout of the date indicia  24 , it only contains a partial layout of the year indicia  26 . A method for laying out additional year indicia  26  is discussed below. 
     In addition, within appropriate segments  18  in the first portion  20  are month ending indicia  28 . Month ending indicia  28  identify on the calendar  10  the end of a given month. Each month ending indicia  28  is placed to coincide with the date indicia  24  indicating the last day of a month. For example, the month ending indicia  28  for April, “Apr,” is positioned in the segment  18  wherein the date indicia  24  represents the 30 th  day. 
     The rotating cap  14  fits over the outer surface  16  of the first portion  20  of the cylindrical body  12 . The rotating cap  14  defines at least one opening  32 . The openings  32  permit the date indicia  24  and the month ending indicia  28  positioned on the outer surface  16  of the cylindrical body  12  to be framed and viewed. As shown in FIG. 1, a preferred embodiment of the present invention includes a rotating cap  14  that defines seven openings  32 , one corresponding to each of the segments  18 . The rotating cap  14  could also be designed such that no openings  32  are required, or that any number of openings  32  are provided. 
     The rotating cap  14  is radially divided into seven sections  34  that are consistent in arc segment with the seven segments  18 . Consistent arc segments for the sections  34  and segments  18  assures that when the rotating cap  14  is repositioned on the cylindrical body  12 , the sections  34  and segments  18  are alignable. Each section  34  includes day indicia  36  and month indicia  38  in a fixed relationship that is appropriately alignable with the fixed relationship between the date indicia  24  and year indicia  26  in each segment  18  of the cylindrical body  12 . FIG. 3 illustrates a complete layout for each section  34 . 
     As indicated above, the layout for the cylindrical body  12  is only a partial layout as it depicts only some number of years. Predominately, the number of years can be increased by following a standard pattern. The standard pattern representing four years has five elements, i.e. x, x+1, x+2, x+3, x+4, x+4. The last two elements are for a single leap year. 
     An example of the pattern as applied to years is as follows—2001, 2002, 2003, 2004, 2004. The next repeat would be 2005, 2006, 2007, 2008, 2008. In the previous patterns, years 2004 and 2008 are leap years. As it is important for operation of the perpetual calendar  10 , which is explained below, to distinguish between the two year indicia  26  for a single leap year the second leap year indicia  26  is highlighted, such as with the letter “L.” 
     This pattern, however, is modified in one unique case. As those who understand calendars appreciate, all centuries are not leap years. A leap year is generally defined as any year divisible by 4. While all centuries are divisible by 4, a century is a leap year only if it is also divisible by 400. Thus the century 2000 is a leap year while the centuries 1900, 2100, 2200 and 2300 are not. In this unique case the pattern is altered by deleting the second duplicate entry. 
     To use the perpetual calendar  10 , the segments  18  of the cylindrical body  12  and the sections of the rotating cap  14  must be properly aligned. In one procedure for using the perpetual calendar, the first step is to determine if the year desired is a leap year or not. As explained above, for each leap year there are two year indicia  2 , e.g. 2004 and 2004L, in adjacent segments. If the year is not a leap year, there is only one year indicia  26 . In the non-leap year case, aligned the desired month indicia  38  with the desired year indicia  26 . For the aligned month and year, a one month calendar will be displayed. In the case of a leap year, if the desired month is January or February align the appropriate month indicia  38  with the first year indicia  26 , e.g., 2004. For all other months in the leap year, align the desired month indicia  38  with the second year indicia  26 , e.g., 2004L. 
     Referring to FIG.  4 . the perpetual calendar  10  can be used to determine the day of the week for a selected date and year. For example, to determine on what day of the week Aug. 17, 2005 will fall, a user positions the rotating cap  14  to align the month indicia  38  portion containing “August” of section  34  with the year indicia  26  containing the year “2005” in segment  18 . The user then views the date indicia  24  through the cap opening  32  to locate the number “17” corresponding to the selected date. The corresponding day indicia  36  indicates “Wednesday”, which allows the user to thereby determine that Aug. 17, 2005 falls on a Wednesday. 
     Although the present invention has been described in considerable detail with reference to a certain preferred versions thereof, other versions are possible, particularly versions wherein the indicia within a segment or section are positioned differently, wherein the openings may not be required or additional openings could be used, or the body and cap are not cylindrical. Therefore, the spirit and scope of the invention should not be limited to the description of the preferred versions contained herein.