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
Bissextus ecclesiastical calendars, the intercalary day is added for convenience at the end of the month of February, as February 29, and years in which February has twenty-nine days are called "bissextile," or leap years. Bissextus Bissext, or bissextus (Lat. "bis", twice; "sextus", sixth) is the day which is added to the Gregorian calendar every fourth year (except those evenly divisible by 100, unless they are divisible by 400) to compensate for the six-hour difference in length between the common 365-day year and the actual length of the solar year. Originally, the day was inserted after the 24th of February, i.e.