The purpose of a calendar is to predict the occurence of important periodic events. According to Foreignword.com, the a calendar is "a system for determining the beginning, length, and order of years and their divisions".

Normally, the basic unit of time in a calendar is the day. In modern terms, the day is the amount of time required for the earth to rotate once around its axis, relative to some object. Different definitions of the day are obtained by the choice of that object. Most astronomers use the mean sidereal day, which is the average time it takes for the earth to rotate once on its axis relative to distant stars that are assumed to be fixed. The mean sidereal day is approximately 86164.1s

The year is also a very important unit of time in calendars. The year is defined as the length of time required for the earth to make a complete orbit around the sun. As with the day, there are several different possible definitions of the year. The tropical year, the year that will be used on this website, is the time between successive vernal equinoxes. Astronomers have measured the tropical year to be approximately 365.24219878 (solar) days. The exact length of the year is in fact constantly shrinking due to tidal friction. A calendar based on keeping in phase with the year is called a solar calendar.

Another important unit of time is the month. A lunation (the month commonly used in calendars) is defined as the amount of time taken for the moon to make one complete revolution around the earth, as measured by the appearance of new moons. A lunation is approximately 29.530588 days. A calendar based on keeping in phase with lunar cycles is a lunar calendar. Because 12 lunations is approximately 354.3671 days, or approximately 10.875 days shorter than a year, a lunar calendar will (unless something additional is done) shift with respect to the seasons every year.

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