Lilian date
A Lilian date is the number of days since the beginning of the Gregorian Calendar
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

 on October 15, 1582, regarded as Lilian date 1. It is named for Aloysius Lilius
Aloysius Lilius
Aloysius Lilius , also variously referred to as Luigi Lilio, Luigi Giglio, or Aluise Baldassar Lilio, was an Italian doctor, astronomer, philosopher and chronologist, and also the "primary author" who provided the proposal that became the basis of the Gregorian Calendar reform of 1582.The crater...

 who devised the Gregorian Calendar. It was invented by Bruce G. Ohms of IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

 in 1986.

Another, better known, date notation that is used for similar purposes is the Julian date. Universal time
Universal Time
Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...

, 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 2299160.5 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.
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