The Julian day number can be considered a very simple calendar, where its calendar date is just an integer.
This is useful for reference, computations, and conversions.
It allows the time between any two dates in history to be computed by simple subtraction.
The Julian day system was introduced by astronomers to provide a single system of dates that could be used when working with different calendars and to unify different historical chronologies.
In his book Outlines of Astronomy, first published in 1849, the astronomer John Herschel wrote: The first year of the current Julian period, or that of which the number in each of the three subordinate cycles is 1, was the year 4713 B.
A more recent starting point is sometimes used, for instance by dropping the leading digits, in order to fit into limited computer memory with an adequate amount of precision.
In the following table, times are given in 24 hour notation.
The Julian day is sometimes referred to as the Geocentric Julian Day (GJD) in order to distinguish it from HJD.
The Julian day number is based on the Julian Period proposed by Joseph Scaliger in 1583, at the time of the Gregorian calendar reform, but it is the multiple of three calendar cycles used with the Julian calendar: Its epoch falls at the last time when all three cycles (if they are continued backward far enough) were in their first year together — Scaliger chose this because it preceded all historical dates.
Now, at , Friday January 12, 2018 (UTC) the Julian day number is 2458131.