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
Leap year lunar months containing 29 or 30 days, generally in alternating order. However, the tabular Islamic calendar used by Islamic astronomers during the Middle Ages and still used by some Muslims does have a regular leap day added to the last month of the lunar year in 11 years of a 30-year cycle. This additional day is found at the end of the last month, Dhu 'l-Hijja, which is also the month of the Hajj. The Hijri-Shamsi calendar, also adopted by the Ahmadiyya Muslim Community, is based on solar calculations and is similar to the Gregorian calendar in its structure with