Calendrical calculation

• Converting a Julian or Gregorian calendar date to its Julian day number and vice versa.
• 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 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.