The number of days since noon on January 1, -4712, i.e., January 1, 4713 BC (Seidelmann 1992). It was proposed by J. J. Scaliger in 1583, so the name for this system derived from Julius Scaliger, not Julius Caesar. Scaliger defined Day One was as a day when three calendrical cycles converged. The first cycle was the 28 year period over which the Julian calendar repeats days of the week (the so-called solar number). After 28 years, all the dates fall on the same days of the week, so one need only buy 28 calendars. (Note that since the Gregorian calendar was adopted the calendar now takes 400 years to repeat.) The second was the 19 year golden number cycle over which phases of the moon almost land on the same dates of the year. The third cycle was the 15 year ancient Roman tax cycle of Emperor Constantine (the so-called indiction). Scaliger picked January 1, 4713 BC on the Julian calendar as Day One. The three cycles coincide every 7980 years (Tøndering).

The following table gives the Julian date for the "zeroth" of each month at 0 UT. So add to get the Julian date of a given day D of the month at a given time .

Formulas for computing the Julian Date from year, month, and day in the Julian and Gregorian calendars are given below. Let denote the integer part (sometimes known in mathematical circles as the floor function ), let Y be the year, M the month number (1=January, 2=February, etc.), D the day of the month, and UT the universal time.

For all AD dates in the Gregorian calendar,

	(1)

For Gregorian calendar dates 1901-2099, the formula can be simplified to

	(2)

(Danby 1988, p. 207; Sinnott 1991, p. 183).

For Julian calendar dates with either negative (BC) or positive (AD),

	(3)

and for positive (AD) Julian calendar dates only,

	(4)

(Sinnott 1991).

Golden Number, Gregorian Calendar, Indiction, Julian Epoch, Julian Year, Modified Julian Date