This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
The Julian calendar was introduced in 45 BC, but when historians date
events prior to that year, they normally extend the Julian calendar
backward in time. This extended calendar is known as the "Julian
Similarly, it is possible to extend the Gregorian calendar backward in
time before 1582. However, this "Gregorian Proleptic Calendar" is
If someone refers to, for example, 15 March 429 BC, they are probably
using the Julian proleptic calendar.
In the Julian proleptic calendar, year X BC is a leap year, if X-1 is
divisible by 4. This is the natural extension of the Julian leap year