The internal mechanism will move the dial to the next day. In watchmaking, "perpetual calendar" describes a calendar mechanism that correctly displays the date on the watch 'perpetually', taking into account the different lengths of the months as well as leap years. Their purpose, in part, is to eliminate the need for perpetual calendar tables, algorithms, and computation devices. Technically, these are not perpetual calendars but perennial calendars. Examples are The World Calendar, the International Fixed Calendar and the Pax Calendar. The current month is turned forward on the front block, with the other two month blocks behind it.Ĭertain calendar reforms have been labeled perpetual calendars because their dates are fixed on the same weekdays every year. In addition to the two cubes, three blocks, each as wide as the two cubes combined, and a third as tall and as deep, have the names of the months printed on their long faces. This is sufficient because only one and two may appear twice in date and they are on both cubes, while the 0 is on both cubes so that all single-digit dates can be shown in double-digit format. The other bears the digits 0, 1, 2, 6 (or 9 if inverted), 7, and 8. One cube carries the digits zero to five. A common device consists of two cubes in a holder. Establishments that serve alcoholic beverages may use a variant that shows the current month and day but subtracting the legal age of alcohol consumption in years, indicating the latest legal birth date for alcohol purchases. Offices and retail establishments often display devices containing a set of elements to form all possible numbers from 1 through 31, as well as the names/abbreviations for the months and the days of the week, to show the current date for convenience of people who might be signing and dating documents such as checks. Other uses of the term "perpetual calendar" 3 Perpetual Julian and Gregorian calendar tables.1 Other uses of the term "perpetual calendar".This is the first known instance of a tabular form of perpetual calendar allowing the calculation of the moveable feasts that became popular during the 15th century. For each year of this period, it lists the number of weeks between Christmas day and Quinquagesima. The calendar covers the period of 1390–1495 (on which grounds the manuscript is dated to c. These issues are dealt with in great detail in computus.Īn early example of a perpetual calendar for practical use is found in the Nürnberger Handschrift GNM 3227a. Such a perpetual calendar fails to indicate the dates of moveable feasts such as Easter, which are calculated based on a combination of events in the Tropical year and lunar cycles. A mixture of the above two variations - a one-year calendar in which the names of the months are fixed and the days of the week and dates are shown on movable pieces which can be swapped around as necessary.The seven calendars may be combined into one, either with 13 columns of which only seven are revealed, or with movable day-of-week names (as shown in the pocket perpetual calendar picture). Some perpetual calendars' tables slide against each other so that aligning two scales with one another reveals the specific month calendar via a pointer or window mechanism. Seven (31-day) one-month calendars (or seven each of 28–31 day month lengths, for a total of 28) and one or more tables to show which calendar is used for any given month.(See Dominical letter for one common naming scheme for the 14 calendars.) These one-year calendars divide evenly into two sets of seven calendars: seven for each common year (the year that does not have a February 29) with each of the seven starting on a different day of the week, and seven for each leap year, again with each one starting on a different day of the week, totaling fourteen.
14 one-year calendars, plus a table to show which one-year calendar is to be used for any given year.One can then deduce the day of the week or the date.Ī perpetual calendar is a calendar valid for many years, usually designed to look up the day of the week for a given date in the past or future.įor the Gregorian and Julian calendars, a perpetual calendar typically consists of one of three general variations: A 50-year "pocket calendar" that is adjusted by turning the dial to place the name of the month under the current year.