During the late Roman Republic, the length of the Roman year and its alignment with the seasons were, as a Roman muli in the legions would say, perfututi… totally screwed up.
In theory, the Roman calendar in use at that time was developed during the reign of the legendary second king of Rome, Numa Pompilius, around 713 BCE. The Roman year had 355 days divided unevenly into twelve months, Ianuarius, Februarius, Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November, December.
In order to keep the calendar aligned with the seasons, the Romans inserted an intercalary month of twenty-two or twenty-three days in alternate years. This mensis intercalaris was typically placed within the month of Februarius (Macrobius, Saturnalia, 1.13.12-5).
That’s the theory, anyway.
In fact, by the time Insubrecus arrives in long-haired Gaul, in The Helvetian Affair, the Roman calendar had lost coordination with the seasons.
The system of aligning the year through the use of intercalary months broke down during the late Republic. The Roman Pontifex Maximus, the high priest, determined when an intercalary month was to occur, which was supposed to happened every other year. However, the position of Pontifex Maximus was held by a member of the Roman political elite. Because the term of office of Roman magistrates was delineated by the Roman calendar year, a Pontifex Maximus could lengthen a year in which political allies were in power, or shorten a year in which political opponents held office.
Julius Caesar, as Pontifex Maximus, reformed the calendar in 46 BCE. The “Julian calendar” had a regular year of 365 days divided into the traditional twelve months. An extra day was added to Februarius every four years to keep the calendar synchronized with the seasons.
In order to realign the calendar with the seasons, 46 BCE was 445 days long, compensating for the intercalary months which had been missed during times of political strife and the chaos of the civil wars. Not surprisingly, this was also a year when Caesar held the consulship.
In order to simplify things, the narrative of the Gaius Marius Chronicle ignores the seasonal confusion of the Roman calendar before the Julian reform. In The Swabian Affair, when Insubrecus worries about being on campaign against Ariovistus along the Rhine in early fall, he identifies the calendar month as September, just as expected.
Roman Designation of Years
During the late Roman Republic, the consular term began with the first day of the Roman year, 1 January.
The years themselves were identified with the consuls in office. For example, in the first book of his Gallic Wars, Caesar dates Orgetorix’ conspiracy among the Helvetians saying, “is M Messala et M Pisone consulibus regni cupiditate inductus coniurationem nobilitatis fecit (while Marcus Messala and Marcus Piso were consuls, [Orgetorix] desired regal powers, so he hatched a plot among the nobles)” (de bello Gallico 1.2). According to modern reckoning, the year was 61 BCE.
Also, during the late Republic, Romans began to count years from the founding of their city. Although Roman historians used different dates for this event, the date most widely used was that of Marcus Terentius Varro, 753 BCE. This Verronian chronology was made official in the reign of Insubrecus’ mentor, patron and erstwhile chum, Augustus.
Dates used according to this method are designated ab urbe condita, “from the founding of the city”, abbreviated AUC. So, the date of the Orgetorix conspiracy mentioned by Caesar occurred in 693 AUC.
One last complication! Romans of course did not use Arabic numerals; they used those pesky Roman numerals which, for some reason beyond my understanding, the entertainment industry still uses to date movies and TV shows. So, 693 AUC becomes DCXCIII AUC.
So, when Insubrecus tells the story of the murdered centurion, which occurred during his first year as the city prefect of Mediolanum, he identifies the year as, anno Consulium Imp Caesaris Divi f Augusti VIII et T Statilii Tauri II AUC DCCXXVIII (During the Consulships of the Imperator, Caesar, Son of the God, the Venerable One, 8th Term, and Titus Statilius Taurus, 2nd Term, 728 years from from the founding of the city). Or, for us moderns, 26 BCE.
In some parts of the imperium, the provincial year, anno provinciarium, or A PP) was used to designate the year. But, this needn’t concern us, since none of the characters in the Gaius Marius Chronicle use this convention.
For those who are not that good at math and have long ago forgotten how to read Roman numerals, the following sites may be helpful:
Convert Roman Numerals: http://www.onlineconversion.com/roman_numerals_advanced.htm