The ancient Romans started the Julian calendar which eventually evolved into the Common calendar. However, the method of the Romans for designating dates in their calendar was quite different from ours. We refer to the Common calendar with the Roman way of designating dates (in the Latin language) as the "Latin Calendar".
The Romans did not know Arabic numerals. They wrote numbers using letters, according to the following table.
Letter Numeric Value I 1 V 5 X 10 L 50 C 100 D 500 M 1000
If a "smaller" letter (i.e., earlier in the table) follows a "bigger"
one (i.e., later in the table), then the values add up, but if a smaller
letter precedes a bigger one, then its value must be subtracted from the
total. For example,
DCX stands for 500 + 100 + 10 = 610, but
CDX stands for 500 - 100 + 10 = 410.
The Latin names for the months are listed in the following table. They are similar to the English month names, which are derived from them. In the Latin language, the way to write a word -- and especially the last part of a word -- depends on the context. The table lists three forms that are useful in the calendar.
Number Latin Month Names English Month Name 1 Ianuarius Ianuariis Ianuarias January 2 Februarius Februariis Februarias February 3 Martius Martiis Martias March 4 Aprilis Aprilibus Apriles April 5 Maius Maiis Maias May 6 Iunius Iuniis Iunias June 7 Iulius Iuliis Iulias July 8 Augustus Augustis Augustas August 9 September Septembribus Septembres September 10 October Octobribus Octobres October 11 November Novembribus Novembres November 12 December Decembribus Decembres December
A month as a whole was referred to with the first of the three listed Latin names. For example, the month of January was mensis Ianuarius.
Three days in each month had names: the Kalends (hence calendar), the Nones, and the Idus (as in "Beware the Ides of March"). The Kalends was the first name of a month. The Idus was the 13th day in most months, but the 15th day in March, May, July, and October. The Nones was 8 days before the Idus, so it was the 5th or 7th day of the month. These days were referred to using month names from the second column of the table; for example Kalendae Ianuariis, Nonae Februariis, Idibus Martiis. The day preceding one of these days was referred to using month names from the third column of the table, after the word Pridie; for example, Pridie Kalendas Apriles, Pridie Nonae Maias, Pridie Idus Iunias.
The Romans indicated other days of the month by counting backwards from the next later Kalends, Nones, or Idus. This means that days in the second half of every month (after the Idus) would be referred to as "so many days before the Kalends of the next month". In addition, the Romans counted inclusive. In figuring out the difference between two numbers, they'd count both the first and the last numbers. For example, to get from today to tomorrow, the Romans would count two days rather than just one. So, the 30th day of June, which is the day before the Kalends (first day) of July, would be referred to as Pridie Kalendas Iulias, and the day before that (the 29th of June) as Ante Diem III Kalendas Iulias. The "ante diem" means something like "the earlier day".
The Romans used to count years from the (mythical) year of the founding of the city of Rome in year -751 of the Common Era. They referred to a year count in the era as Ab Urbe Condita ("since the founding of the City"), abbreviated to A.U.C. However, our Latin calendar uses the same era as the Common calendar. The year number is introduced by the word "Anno" (year). As an example of a complete date, the 15th of December of 1965 is referred to as "Ante Diem XVIII Kalendas Ianuarias Anno MCMLXVI", which translates loosely as "The 18th inclusive day before the Kalends of January of the year 1966".
The Romans did not know of the number zero or of negative numbers. Such year numbers are printed in the Latin calendar using the usual Arabic numerals. In addition, numbers greater than or equal to 4000 are also printed using Arabic numerals.