When using the Latin numbering system, which does not include zero, it is traditional to represent the years preceding 1 as "1 BC" etc. In this system the year 1 BC is a leap year (likewise in the proleptic Julian calendar).
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 Gregorian calendar". In this system the year 0 is a leap year.
Note that because around this time the Julian Calendar was in actual use, historians and astronomers prefer to use the Julian Calendar.
Likewise, the Proleptic Julian Calendar is used to specify dates before its official introduction in 45 BC.