Explainer: Latin Months and Roman Dates
Kalends, Nones, Ides, and Ante Diem
If you begin reading Latin inscriptions, chronicles, or early modern texts, you will quickly encounter dates that look unfamiliar.
XIII KAL IANVAR
IDIB DECEMBR
a.d. IV NON MART
These are not oddities or abbreviations gone wrong. They belong to a coherent system of dating that Romans used for centuries, and which continued in Latin long after antiquity.
This post explains:
the Latin names of the months
the Roman system of Kalends, Nones, and Ides
how the ante diem formula works
why dates are counted backwards
and why Ides often appears as Idib. in inscriptions
The Latin Months of the Year
Here are the classical Latin names of the months:
January — Ianuarius
February — Februarius
March — Martius
April — Aprilis
May — Maius
June — Iunius
July — Iulius
August — Augustus
September — September
October — October
November — November
December — December
In dates, month names usually appear:
in the genitive case (“of January”), and
in abbreviated forms, especially in inscriptions (IANVAR, DECEMBR, etc.).
The Three Fixed Points of the Roman Month
Romans did not number days sequentially. Instead, each month was organized around three fixed reference points.
Kalends (Kalendae)
Always the first day of the month
Source of the word calendar
Kal. Ian.
= January 1
Nones (Nonae)
Fall on:
March, May, July, October → the 7th
All other months → the 5th
Ides (Idus)
Fall on:
March, May, July, October → the 15th
All other months → the 13th
The famous Idūs Mārtiās are March 15.
The Ante Diem Formula
Most Roman dates use the ante diem system.
ante diem means:
“before the day”
A Roman date tells you how many days before the next Kalends, Nones, or Ides the date falls. The counting is always inclusive, meaning both the target day and the reference day are counted.
The full form looks like this:
ante diem tertium Kalendas Aprilis
But it is almost always abbreviated:
a.d. III Kal. Apr.
Often, ante diem is omitted entirely:
III Kal. Apr.
The logic remains the same.
How Inclusive Counting Works
This is the part that confuses most learners.
Romans count:
backwards
including the final day
This makes Roman dates feel “off by one” until you adjust your perspective.
Examples with Kalends
Example 1
III Kal. Apr.
Kalends of April = April 1
Counting backwards inclusively:
Count 1 → April 1
Count 2 → March 31
Count 3 → March 30
✅ III Kal. Apr. = March 30
Example 2
VIII Kal. Sept.
Kalends of September = September 1
✅ VIII Kal. Sept. = August 25
Examples with Nones
Example 1
a.d. IV Non. Mart.
Nones of March = March 7
✅ a.d. IV Non. Mart. = March 4
Example 2
Prid. Non. Mai.
pridie means “the day before”
Nones of May = May 7
✅ Prid. Non. Mai. = May 6
(Pridie is extremely common and avoids counting.)
Examples with Ides
Example 1
V Id. Apr.
Ides of April = April 13
✅ V Id. Apr. = April 9
Example 2
Idib. Decembr.
✅ Idib. Decembr. = December 13
This leads to an important grammatical point.
Why Ides Appears as Idib.
You may expect Ides to be abbreviated simply as Id., but inscriptions often use Idib. instead.
This is because Idus is a plural noun, and Roman dates frequently use it in the ablative plural to express time when.
The full form is:
Idibus Decembris — on the Ides of December
So:
Idus = the Ides (nominative plural)
Idibus = on the Ides (ablative plural)
Idib. = standard abbreviation of Idibus
This is not a special calendrical trick—it is simply normal Latin grammar written compactly for stone.
Why Romans Used This System
Roman dates answer a different question than modern ones.
Modern dates ask:
“What day of the month is it?”
Roman dates ask:
“How far are we from the next important day?”
Kalends, Nones, and Ides were tied to:
religious observances
legal deadlines
public announcements
They were social and civic reference points, not arbitrary numbers.
Why This Still Matters for Latin Learners
If you read:
inscriptions
medieval chronicles
Neo‑Latin texts
early modern documents
you will encounter this system constantly.
It survived for centuries because it worked—and because tradition matters deeply in Latin writing.
Once you recognize Kalends, Nones, Ides, and ante diem, Roman dates stop being mysterious and become one of the most distinctive and expressive features of historical Latin.