This tag is for questions about combinatorial and computational issues with regards to calendar systems. A typical, well-known mathematical result in this direction is John Conway's Doomsday Algorithm for computing the "day of the week" associated to a given date.

Calendar computations have a long history within mathematics. In fact, much of ancient mathematics (such as those developed by the Babylonians, Mayans, and Chinese) stem from an effort to try to understand the mathematical relationships (especially the quasi-periodic nature) between the solar year, the lunar month, and the sidereal day.

With the development of the Gregorian calendar, in modernity the academic interest in calendar computations have somewhat waned. But there are still many interesting recreational problems related to the computations of dates, the interaction between lunar and solar calendars, and relationship between the human concept of the week with more astronomically based measurements of time.