Proleptic julian calendar

The proleptic Julian calendar is produced by extending the Julian Calendar to dates preceding its official introduction in 45 BCE.

When using the Latin numbering system, which does not include zero, it is traditional to represent the years preceding 1 as "1 BCE" etc. In this system the year 1 BCE is a leap year.

When using a numbering system which includes zero, it is more convenient to include a year zero and represent earlier years as negative. This is the convention used in the "astronomical Julian calendar". In this system the year 0 is a leap year.

Likewise, the Proleptic Gregorian Calendar is used to specify dates before its official introduction in 1582. Because the Julian Calendar was actually used before that time, one must explicitly state that a given date is in the Proleptic Gregorian Calendar when that is used.