Proleptic Julian calendar (AUC or by reference to consuls). The year 46 BC itself is a special case, because of the historical introduction of the Julian calendar in that year, it was allotted 445 days. Before then, the Roman Republican calendar used a system of intercalary months rather than leap days, but the Julian system of leap days had already been in use since 238 BC in Ptolemaic Egypt, since the Decree of Canopus reforming the Egyptian calendar. Proleptic Julian calendar The proleptic Julian calendar is produced by extending the Julian calendar backwards to dates preceding AD 4 when the quadrennial leap year
Calendar year length of 365.25 days, and the Hebrew calendar has an average length of 365.2468 days. The Islamic calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days. The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the year in most calendars, but the astronomer's value changes over time, so William Herschel's suggested correction to the Gregorian calendar may become unnecessary by the year 4000. The calendar year can be divided into four quarters, often abbreviated as Q1, Q2, Q3,