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The Roman calendar organised festivals, court business and public activity as well as the passage of time. Its form changed: the Republican calendar with an intercalary month must be distinguished from the Julian calendar introduced under Caesar. Romans commonly dated days in relation to the Kalends, Nones and Ides, and identified a year by its serving consuls.
The developed Republican calendar had twelve months. September, October, November and December retained names derived from seven, eight, nine and ten, although their positions differed in a year beginning with January. Roman tradition credited Romulus with a ten-month calendar and Numa with reforms. These early accounts survive in much later writers, and their details remain disputed.
From 153 BCE consuls entered office on 1 January. This should not be treated as the single origin of every Roman New Year convention: civic dating, terms of office and religious cycles had different histories. Quintilis and Sextilis were later renamed Julius and Augustus.
The Kalends fell on the first day of each month. In March, May, July and October the Nones fell on the seventh and the Ides on the fifteenth; in the other months they fell on the fifth and thirteenth. Intermediate dates were counted backwards to the next fixed day, including both the starting and ending days.
For example, 13 March was the third day before the Ides of March: the inclusive count comprises 13, 14 and 15 March. The fourteenth was pridie Idus Martias, the day before the Ides. Simply subtracting the numeral therefore gives the wrong date. Roman numerals help in reading abbreviated dates; a.d. here means ante diem, not Anno Domini.
An ordinary Republican year contained 355 days. An intercalary month was used to bring it into line with the solar year, under pontifical supervision. Irregular intercalation could cause civil dates to drift away from the seasons.
Julius Caesar's reform established a solar year of 365 days with a periodic extra day, taking effect in 45 BCE. Early mistakes in applying the leap-year rule were later corrected under Augustus. Roman dating associated the extra day with repeating a day before the Kalends of March, hence bissextilis. The modern label "29 February" does not literally reproduce that ancient method.
Surviving calendars, or fasti, mark festivals and categories of days. F, N and C concern permissible forms of judicial and public activity; they are not straightforward equivalents of modern working days and weekends. The nundinal cycle, labelled A–H, was associated with market days. The planetary seven-day week became widespread later.
A year could be identified by the names of its consuls. Dating from Rome's foundation is familiar from historical writing, but was not the sole everyday standard. Imperial inscriptions may be dated through consulships, tribunician powers and other elements of a ruler's titulature. A numeral in one position does not necessarily mean the same thing as a numeral elsewhere.
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