Century leap year was introduced in 1582, but was adopted by various countries over several centuries, with the result that some countries still used the older Julian calendar while others used the Gregorian calendar. Dates prior to 1582 are generally calculated using the Julian calendar, and different countries have different conventions about dates between 1582 and their adoption of the Gregorian calendar. (See, for example, Old Style and New Style dates.) Century leap year In the Gregorian calendar, a year ending in "00" that is divisible by 400 is a century leap year, with the intercalation of February 29 yielding 366 days instead
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