Calendar

===Arithmetical and astronomical===
[[File:JudischerKalender-1831 ubt.jpeg|thumb|Calendar of the [[Qahal]], 5591 (1831)]]

An ''astronomical calendar'' is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the [[Second Temple]]. Such a calendar is also referred to as an ''observation-based'' calendar. The advantage of such a calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult.

An ''arithmetic calendar'' is one that is based on a strict set of rules; an example is the current [[Jewish calendar]]. Such a calendar is also referred to as a ''rule-based'' calendar. The advantage of such a calendar is the ease of calculating when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation. This limits the lifetime of an accurate arithmetic calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar.