Proleptic Julian calendar
The
proleptic Julian calendar is produced by extending the
Julian calendar to dates preceding its official introduction in
45 BC.
A calendar obtained by extension earlier in time than its invention or implementation is called the "proleptic" version of the calendar, and thus we obtain the proleptic Julian calendar. Likewise, the
proleptic Gregorian calendar is occasionally 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.
Historians since
Bede have traditionally represented the years preceding AD 1 as "1 BC", "2 BC", etc. Bede and later Latin writers chose not to place the Latin zero,
nulla, between BC and AD years. In this system the year 1 BC would be a leap year (although the leap years actually observed between 46 BC and AD 4 were erratic: see the
Julian calendar article for details). To determine an interval in years across the BC/AD boundary, 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 equivalent to 1 BC and is a leap year.
* Cited from:
The Julian and Gregorian Calendars hosted by
Hermetic Systems*
Julian date
