A Lilian date is the number of days since the beginning of the Gregorian Calendar on October 15, 1582, regarded as Lilian date 1. It is named for Aloysius Lilius who devised the Gregorian Calendar. It was invented by Bruce G. Ohms of IBM in 1986.
Another, better known, date notation that is used for similar purposes is the Julian date. Universal time, or closely related time scales, are always used with the Julian date, the day begins at noon, and a decimal fraction may be used to represent the time of day. In contrast, Ohms did not make any mention of time zones or time of day in his paper; the Lilian date is a pure date format. If time is reckoned in Universal Time, and the Lilian date is taken to begin at midnight, the Lilian date can be obtained from the Julian date by subtracting 2,299,159.5 from the Julian date and ignoring the decimal fraction in the result. Lilian dates can be used to calculate the number of days between any two dates occurring since the beginning of the Gregorian calendar. It is currently used by date conversion routines that are part of IBM Language Environment (LE) software.
- Ohms, Bruce G (1986). "Computer processing of dates outside the twentieth century". IBM Systems Journal (IBM) 25 (2): 244–251. doi:10.1147/sj.252.0244.
- "CEEDATE—convert Lilian date to character format". COBOL for AIX Programming Guide (1st ed. ver. 2). 2004.
- Date and Time Services in z/OS V1R12.0 Language Environment Programming Reference