Proleptic Julian calendar with year 0 corresponding to 1 BC, and "year −1" corresponding to 2 BC, so that in this system, Julian leap years have a number divisible by four. The determination of leap years in the proleptic Julian calendar (in either numbering) is distinct from the question of which years were historically considered leap years during the Roman era, due to the leap year error: Between 45 BC and AD 4, the leap day was somewhat unsystematic. Thus there is no simple way to find an equivalent in the proleptic Julian calendar of a date quoted using the Roman pre-Julian calendar
365-day calendar length of 365 days with no rule for leap days, despite potential leap year rules being acknowledged by the 9th century at the latest. In particular, of the versions still in use today the Qadimi version does not have any form of leap rule; the Shahanshahi version had one leap month added in the 12th century but no leap years since, while the Fasli version (introduced in the 20th century) adds one day every four years. 365-day calendar A 365-day calendar consists of exactly 365 days per year (no leap days), and is primarily used in computer models and as