"If there is discord among the wisest men, can it astonish us that the civil years, which were established by less accomplished persons, differed one from the other and corresponded but badly with the natural years?"
Censorinus, De Die Natale (VIII)
With the expulsion of Tarquinius Superbus, the last Etruscan king, and the establishment of the Roman Republic in 509 BC (245 AUC), supreme power (imperium) resided in two consuls, who were elected annually. From 222 BC they assumed office on March 15 (the Ides of March), just before the vernal equinox. It was spring, and March, named after Mars, the god of war, was the start of the military campaign season. In 153 BC, however, consuls began to assume power on January 1, which now marked the beginning of the consular or civil year as well as the calendar year. (This is not to say that the Kalends of January became the New Year on that date. The first crescent moon after the winter solstice, when light begins to increase over darkness, likely marked the natural beginning of the year even when it had begun in March.)
Why the consular year, at least, began on January 1 was due to the Second Celtiberian War. In 154 BC, there was rebellion in Spain. Quintus Fulvius Nobilior was designated consul for the following year but could not assume office until the Ides of March. Given the military situation, the Senate decreed January 1 to be the start of the new civil year, which permitted Nobilior to be inducted and depart with his legions that much sooner. He still was delayed in arriving, however, as can be determined by a severe defeat late in August, a loss so disastrous that the day on which it occurred was declared a dies aster and subsequently was considered unlucky. Indeed, Appian relates that no Roman general would willingly initiate a battle on that day.
Even after Varro had posited the traditional founding of Rome (April 21, 753 BC, in the third year of the sixth Olympiad) as an historical point from which subsequent years could be counted, Romans tended to date past events, not from the foundation of the city (ab urbe condita, abbreviated AUC), which was not agreed upon in any event, but by the consular year as designated by the names of the consuls then in office. The assassination of Julius Caesar occurred, not in 710 AUC (44 BC), but in the fifth year of his consulship and the first year of Mark Antony's. This eponymity continued for more than twelve hundred years, until, in AD 537, Justinian I ended the consulship, which would be held by the emperor. Dating now would be by the regnal year. The consular record (fasti consulares), which had been recorded from the beginning of the Republic, ended in the West in AD 535 and in the East in AD 54.
The relative chronology of the consular year still had to be synchronized to some absolute point in the past whereby Romans could orient themselves in time. For Rome, that event was the sack of the city by the Gauls in 390 BC, the earliest fact of Roman history, as dated by contemporary Greek authors (cf. Plutarch, "However, it would seem that some vague tidings of the calamity and capture of the city made their way at once to Greece" (Life of Camillus, XXII).
Discovered in the mid-sixteenth century and now in the Palazzo dei Conservatori (Rome), the Fasti Capitolini Consulares et Triumphales list the consuls from 483 to 19 BC and triumphators from 753 to 19 BC. Originally, it may have decorated the Arch of Augustus, which was erected in 19 BC to celebrate the return of the standards that had been captured from Crassus by the Parthians at Carrhae (Dio, LIV.8.1-3). Or it may have adorned the Regia near where the the fasti were found.
In the detail above, the Bellum Philippicum (Second Macedonia War), which began in 200 BC, is noted in the center of the first line. Then the two consuls for that year are named. On line eight, Marcus Porcius Cato and Lucius Valerius Flaccus are listed as consuls (195 BC); on the next line, Publius Cornelius Scipio Africanus and Tiberius Sempronius Longus (194 BC). Then, slightly indented, are the names of the censors and the number of times (XXXXVII) the lustrum had occurred (the purification rite performed every five years after the census). On the next line, in the left-hand margin, the number DLX is inscribed, that is, the 560th year since the foundation of Rome (here calculated from 752 BC) or 193 BC.
Feeney argues that, by adding the number of years from the founding of Rome to the consular fasti, Augustus subtly but profoundly altered how civil time was apprehended. The list of consular eponyms begins to lose its significance in reckoning time and, instead of identifying the year, the importance of their succession devolves simply to a place in a larger chronology. Although the consular year still was in place, by the end of Augustus' reign, its symbolic importance has been completely redrawn.
Reference: Caesar's Calendar (2007) by Denis Feeney.
