A calendrical calculation is a calculation concerning calendar dates. Calendrical calculations can be considered an area of applied mathematics. Calendrical calculation is one of the five major Savant syndrome characteristics. 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.