The proleptic Julian calendar is produced by extending the Julian calendar to dates preceding its official introduction in 45 BC.
Historians since Bede have traditionally represented the years preceding AD 1 as "1 BC", "2 BC", etc. 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). (Bede and later Latin writers chose not to place the Latin zero, nulla, between BC and AD years.)
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 (equivalent to 1 BC) 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.
See also: