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
Mesoamerican Long Count calendar 206, which corresponds to the year 600 AD. Next, divide this number by 365 days (vague year). The remainder is 44.86849 years, which is 44 years and 317 days. The full year date is 644 AD. Now calculate the month and day number, taking into account leap days over the 44 years. In the Gregorian Calendar, every fourth year is a leap year with the exception of centuries not evenly divisible by 400 (e.g. 100, 200, 300). When the year is divisible by 400 (e.g. 400, 800, etc.), do not add an extra day. Our calculated year is 644 AD.