The usual method of expressing dates in days, months, years, hours, minutes, and seconds can be tricky to deal with mathematically. If you try to figure out the difference in days between July 4, 1776 and today’s date, the problem becomes evident. To avoid this, astronomers often express time in terms of Julian Day when making calculations, with decimal fractions corresponding to fractions of a day. JD 0.0 corresponds to January 1, 4713 BC; any other JD is the number of days since then. Thus, January 1, 2000 was JD 2451545.

This system is unrelated to the Julian calendar. The name "Julian" Day comes from Joseph Scaliger who devised the system in 1583 and named it after his father, Julius. Scaliger proposed a period of 7,980 years of numbered days to be used in determining time elapsed between various historical events otherwise recorded only in different calendars. The length of 7,980 years was chosen as the product of 28 times 19 times 15; these, respectively, are the numbers of years in the solar cycle of the Julian calendar in which dates recur on the same days of the week; the lunar or Metonic cycle, after which the phases of the Moon recur on a particular day in the solar year; and the cycle of indiction, originally a schedule of periodic taxes or government requisitions in ancient Rome. The epoch, or starting point, of 4713 BC was chosen as the nearest past year in which the three cycles began together.