Calendrical calculation

A calendrical calculation is a calculation concerning calendar dates. Calendrical calculations can be considered an area of applied mathematics. Some examples of calendrical calculations:

Converting a Julian or Gregorian calendar date to its Julian day number and vice versa (see the section on calculation in that article for details).
The number of days between two dates, which is simply the difference in their Julian day numbers.
The date of a religious holiday, like Easter (the calculation is known as Computus) or Passover, for a given year.
Converting a date between different calendars. For instance, dates in the Gregorian calendar can be converted to dates in the Islamic calendar with the Kuwaiti algorithm.
Calculating the day of the week.

References

Edward M. Reingold and Nachum Dershowitz. Calendrical Calculations. Cambridge University Press; 3 edition (December 10, 2007). ISBN 978-0-521-88540-9