The result of an astronomical calculation is sometimes an instant of time, expressed as a number of days since a fundamental epoch. In our convention, that epoch is 1900 January 0.5, and the number of days elapsed since then is represented by the real variable DJ. For example, we may have calculated the time of the next lunar eclipse as DJ = 30 815.69, but unless we are familiar with the interpretation of such numbers, the result will not mean very much to us. We need a conversion routine to convert a Julian day number into a calendar date, the inverse function of JULDAY.

Subroutine CALDAY performs this function. It converts DJ, the number of Julian days (and fraction of a day) elapsed since 1900 January 0.5, into the calendar date represented by DY, the day of the month including the fraction of the day, MN, the month number, and YR, the year number. The fraction of the day is also returned by FD. Thus DJ = 30815.69 gives DY = 15.19, MN = 5, YR = 1984 and FD = 0.19, from which we conclude that the lunar eclipse was in progress on 1984 15th May at 4 : 33 UT (Universal Time). The integer part of the day number is returned in the variable ID, and a string representation of the entire date in the string variable DT$.