Ted's Museum: The Roman Calendar
Today is

A Brief History

In ancient times, the calendar was based on the cycles of the moon, which did not agree with the solar year and the four seasons. Ancient societies tried different ways to amend the calendar in order to reconcile lunar and solar years.

The Romans inherited the calendar by way of the Greeks. The calendar of Republican Rome had only ten months and not enough days. Martius, or March, was the beginning of the Roman year. Januarius and Februarius were added later.

Julius Caesar, in his role as Pontifex Maximus, was responsible for refining the calendar into essentially the same form we use today. For quite a while, however, the calendar retained its lunar terminology.

The months in latin are:

On this page, I used Roman numerals instead of the full numbers. Normally, Roman numerals are used for the abbreviated form of the date.

The ordinal numbers in latin (in the accusative) from 1 to 20 are:
 tertium decimum
 quartum decimum
 quintum decimum
 sextum decimum
 septimum decimum

How it Worked

The three important days in the month are the Kalends, the Nones, and the Ides. All other dates in the month are measured by their proximity to these points.

Originally, the pontiffs announced the New Moon on the Kalends. This is the first day of every month.

The Ides were originally the day of the Full Moon. This day is usually the 13th day of the month, except in the months of March, May, July, and October, when it is the 15th ("Beware the Ides of March").

The Nones occured when the moon reached its first quarter, or nine days (as the Romans counted) before the Ides. This day is usually on the 5th, except in the months of March, May, July, and October, when it is the 7th.

If the date is one of these days (let's call them date points), it is expressed in the ablative case. For example:

Kalendis Januariis
Nonis Februariis
Idibus Martiis
January 1
February 5
March 15

The day before one of these date points is expressed with pridie (the day before) and the day in the accusative case. For example:

pridie Kalendas Januarias
pridie Nonas Februarias
pridie Idus Martias
December (yes, December!) 31
February 4
March 14

Now for all the other days. These days are expressed in relation to the next date point. Examples of this format are "3 days before the Ides" or "16 days before the Kalends".

When counting to the next date point, the Romans included the date point, as well. Thus, January 11 is 3 days before the Ides (January 13), since the Ides is included in the count. The format in Latin is "ante diem" plus the number of days plus the date point in the accusative case. For example:

ante diem III Kalendas Januarias
ante diem IV Nonas Februarias
ante diem V Idus Martias
December 30
February 2
March 11

Note how any date after the Ides is expressed by the number of days before the Kalends of the next month.

Cardinal numbers are: one, two, three... Ordinal numbers are: first, second, third...

Romans used ordinal numbers in the date reckoning, as numeral adjectives connected to the word "diem". Since diem in this context is in the accusative case, the number will also be accusative. In other words, they end in -um and not -us.

Let's Make a Date

Here's how to convert any date into the Roman calendar (if it's not the Kalends, Nones, or Ides, or the day before). For dates before the Ides or Nones:
  1. Determine the next date point (Kalends, Nones, or Ides).
  2. If the point is the Nones or Ides, add 1 to that date.
  3. If the point is the Kalends of the next month, add 2 to the total days of the current month.
  4. Subtract the current date from this number.

For example, to get the Roman date for September 9:

  • The next date point is the Ides of September = 13.
  • Add 1 to 13 = 14.
  • Subtract 9 (our date) from 14 = 5.

The Roman date is ante diem V Idus Septembres, or 5 days before the Ides of September.

To get the Roman date for July 21:

  • The next date point is the Kalends of August.
  • Add 2 to 31 (the number of days in July) = 33.
  • Subtract 21 (our date) from 33 = 12.

The Roman date is ante diem XII Kalendas Augustas.

Leap Years

Now for some hard stuff. The Roman calendar also included leap years. However, instead of adding an extra day at the end of February, like we do (February 29), February 24 was counted twice. This doesn't quite agree with our system, but for the purposes of converting, the dates between February 14 and 23 are the same as in a non-leap year. So, don't include February 29 when counting to the next kalends.

February 24 is ante diem VI Kalendas Martias and February 25 is ante diem VI Kalendas Martias bis. This is more of an extension of February 24 rather than two separate days.

February 26 to 29 are then counted like February 25 to 28 in a non-leap year.

Why February? Remember that March was originally the first month of the year, so they added the extra leap day at the end of February, the last month of the Roman year.


Some famous Romans and me

Questions? Comments? Reach me at: