This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
The Julian period (and the Julian day number) must not be confused
with the Julian calendar.
The French scholar Joseph Justus Scaliger (1540-1609) was interested
in assigning a positive number to every year without having to worry
about BC/AD. He invented what is today known as the "Julian Period".
The Julian Period probably takes its name from the Julian calendar,
although it has been claimed that it is named after Scaliger's father,
the Italian scholar Julius Caesar Scaliger (1484-1558).
Scaliger's Julian period starts on 1 January 4713 BC (Julian calendar)
and lasts for 7980 years. AD 2003 is thus year 6716 in the Julian
period. After 7980 years the number starts from 1 again.
Why 4713 BC and why 7980 years? Well, in 4713 BC the Indiction (see
section 2.14), the Golden Number (see section 2.12.3) and the Solar
Number (see section 2.4) were all 1. The next times this happens is
15*19*28=7980 years later, in AD 3268.
Astronomers have used the Julian period to assign a unique number to
every day since 1 January 4713 BC. This is the so-called Julian Day
(JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC
to noon UTC on 2 January 4713 BC.
This means that at noon UTC on 1 January AD 2000, JD 2,451,545
This can be calculated thus:
From 4713 BC to AD 2000 there are 6712 years.
In the Julian calendar, years have 365.25 days, so 6712 years
correspond to 6712*365.25=2,451,558 days. Subtract from this
the 13 days that the Gregorian calendar is ahead of the Julian
calendar, and you get 2,451,545.
Often fractions of Julian day numbers are used, so that 1 January AD
2000 at 15:00 UTC is referred to as JD 2,451,545.125.
Note that some people use the term "Julian day number" to refer to any
numbering of days. NASA, for example, uses the term to denote the
number of days since 1 January of the current year.