Page "Proleptic Julian calendar" Paragraph 3

The Roman calendar was somewhat erratic especially before 45 BC.

The best-known example is the year 46 BC: as part of the reform that initiated the Julian calendar of that name, 46 BC was allotted 445 days by Julius Caesar.

Before then, the Romans added whole intercalary months in an unsystematic way.

Between 45 BC and AD 4, the leap day was also 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 ( AUC or by reference to consuls ).