Roman dates were calculated "from the founding of the city" of Rome, or ab urbe condita (AUC). This was assumed to be 750 BC, although calculations by Marcus Terentius Varro in the 1st century BC determined 753 BC to be the founding date. An alternative system had become more common even by Varro's time, whereby the Romans referred to the names of the consuls rather than the date of the year. References to the year of consulship were used in both conversation and official records. There were two consuls at any one time, and sometimes they each held several terms, meaning that one had to be well educated in history to understand the references. The two systems were compatible; so that the consulship of Quintus Fufius Calenus and Publius Vatinius could be determined as 707 AUC (or 47 BC), the consulship of Caius Julius Caesar (III) and Marcus Aemilius Lepidus as 708 AUC (or 46 BC), and the consulship of Caius Julius Caesar (IV) as 709 AUC (or 45 BC).
The Romans had an eight day week, with the market-day falling every eight days. The old Roman year had 304 days divided into 10 months, beginning on XI Kal. Maius, or 21 April. The extra months Ianuarius and Februarius had been invented as stop-gaps. Julius Caesar realised that the system had become inoperable, so he effected drastic changes in the year of his third consulship. The New Year in 709 AUC began on 1 January and ran over 365 days until 31 December. Further adjustments were made under Augustus, who introduced the concept of the "leap year" in 737 AUC (AD 4). The resultant Julian calendar remained in almost universal use in Europe until 1582.
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