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The Roman calendar's unusual feature is a day identification by inclusive counting up to a coming month event. The Roman calendar had 3 special monthly events: calends, nones and ides. So three days of the month were named after these events, e.g., Ides of March or Nones of April or Kalends of May. All other days of the month were identified by counting days up to one of three events, e.g., 10 days before the Kalends of June. Similar to other ancient solar calendars, the Roman calendar was inherited from a similar lunar calendar. Initially, the kalends coincided with the new moon, the nones - with the first quarter, ides - with the full moon. Over time, the calendar was aligned with the solar year, and the original significance of these events was lost, but the kalends, ides, and nones remained an integral part of the Roman calendar.
Until March 1st, 4 С.E., the Roman calendar system has changed frequently (see the brief history of the calendar for the calculator), so do not rely on an exact date matching up to this point.
The calculator that converts a date from the Gregorian calendar to the Roman one:
Kalends, Nones, Ides
The kalends was always the first of the months, the ides was the middle of the month, and the nones was in between. The ides of March, May, July, or October fall on the 15th day of the month, the ides of other months fall on the 13th. The nones were always 8 days before the ides. Other days were identified by counting the days up to and including the next event (Ides, Nones or Kalends), e.g., Ante Diem III Idus Aprilis, is 3 days (inclusive) before the ides of April or April 11th (because the ides of April falls on 13th, 13-(3-1) = 11). The day immediately before the event was named with the word "pridie" + event, e.g., Pridie Idus Aprilis means the day before the ides of April.
In Latin, three dates (the Kalends, the ides, and the nones) were written in the ablative case, such as Idibus Aprilibus - the April ides. The days preceding these dates are used in the accusative case, for example, Ante Diem III Idus Aprilis - 2 days before the April ides.
After the calendar reform of Julius Caesar, a leap day was inserted after the 6th day preceding the March calendars and was designated the same as the previous one, only with the addition of bis, e.g., a.d. bis VI Kal. Mar. The alternate name of the leap day - bissextile came from this particular system of designating the day bis sextus - the second sixth.
Roman calendar years counting
The calculators count the years from the founding of the city of Rome - Ab Urbe Condita (A.U.C.). The Rome foundation date is assumed as 753 B.C.E.1 Some sources mention other founding dates; therefore, our calculators allow you to change these parameters (switch on 'Show settings'). Actually, this counting method was not widely used in Ancient Rome; instead, the years were counted from the year of the consul's reign.
Roman calendar brief history
The early Roman calendar had 10 months, the first was March, and the last December (December translated from Latin - tenth), later January and February appeared. The length of the ordinary Roman year before the reform of Julius Caesar was 355 days. To match the solar year, the pontiffs of ancient Rome periodically inserted an extra month into the calendar. Doing a particular sequence of the insertions of the extra month, this problem could be solved. However, as historical documents show, the leap month insertion was unsystematic, which created confusion and misunderstanding.
The calendar mess reached its apogee during the reign of Caesar, who solved the problem by increasing the duration of the months and introducing an additional day in the interval between the February ides and March calendars once every four years. The reform was completed in 45 BC and 44 BC. Julius Caesar was killed. After his death, the former 5th month (Quintilis) became known as Iulius in his honor.
Perhaps because of the Roman strange habit of counting dates inclusively, Caesar’s decree was misinterpreted, and leap days were added not every four years but every three during 36 years.
Fortunately, Emperor Augustus corrected the situation by stopping the introduction of leap years over the next 12 years. Even during Augustus's reign in 8 CE, the former sixth month (Sextilis) was renamed Augustus in his honor. Some of the rulers of Rome following Augustus also tried to rename the months in honor of themselves and their relatives, but their innovations have not reached our days, apparently because they did not change anything except the month names in the calendar.
A calculator setting allows you to set the way the renamed months are displayed. By default, the display depends on the date, up to 43 B.C.E. and 8 C.E. July and August will respectively be shown under the old name (Quintilis and Sextilis).
N.Dershowitz, E.M. Reingold Calendrical Calculations Third edition. ↩