The Roman Dating System

Kalendae, Nonae, Idus and their corresponding lunar phases Kalendae, Nonae, Idus and their corresponding lunar phases

The Romans counted the days of each month in a somewhat confusing manner. This stemmed from the fact that the ancient Roman calendar was, above all, interested in lunar observations. In order to respect the changes that occurred between each phase of the moon, each month was given three fixed days with their own name. The first day of the month was called the kalends (kalendae in Latin; this likely explains the origin of the word “calendar” in English). On this day, the pontiff declared the start of the new moon and the cycle was given an official start date. The next fixed day was the nones (Nonae in Latin), which was announced on the first quarter (by which they meant the first “phase”). The third and last fixed day was the ides (Idus in Latin), or what is known to us today as a full moon. You may be wondering what happened to the last quarter: the Romans simply ignored this last phase of transition where the moon reverts back to its new moon phase. For months that originally possessed 31 days (Martius, Maius, Quintilis - later Iulius and October), the nones was set as the 7th day and the ides was on the 15th. For the rest of the months of the year, the nones was established as the 5th day while the ides moved to the 13th. After later changes to the calendar were made, it no longer became possible to keep the calendar and the moon phases in check. Not wanting to remove the Kalendae, Nonae, and Idus, accommodations were made to retain these fixed dates.

To understand these adjustments, first one needs to be aware of a couple features in the Latin language. The first thing to know is that their prepositions fall under two categories: accusative and ablative. The second thing to know is that beyond the surface understanding of each preposition, these two categories convey a sense of moving "towards/forward" (accusative) or "away/backwards" (ablative). For example, while the Latin preposition post may functionally be used to describe something that comes "after" something else, because it's an accusative preposition, it also conveys a positive, forward-moving sense. So, when describing a date that occurred one day before the kalends, nones, or ides, the Romans opted for the accusative pridie (it was not common for them to describe dates from the perspective of after these fixed dates, but when they did, they used the accusative postridia). The rest of the days of the month were not given a name. Instead, they would simply count the number of days until the next fixed date, and preface that number with the accusative ante diem (“before the day”). It should be noted that kalends, nones, and ides are always used in the plural in English as it is in Latin (this is why we add “the” before using any of them in a sentence, such as in “the ides of March”).

Language rules for months
English nameLatin nameAccusativeAblative
JulyIulius (Quintilis)Iulias (Quintiles)Iuliis (Quintilibus)
AugustAugustus (Sextilis)Augustas (Sextiles)Augustis (Sextilibus)

Some examples:

  • Kalendis Ianuariis (ablative, “the kalends of January”) is the 1st of January
  • Nonis Februariis (ablative, “the nones of Februrary”) is the 5th of February
  • pridie Idus Martias (accusative, “the day before the ides of March”) is the 14th of March
  • ante diem VI Nonas Iulias (accuasitve, “six days before the nones of July”) is the 2nd of July
  • a.d. XII Kalendas Aprilis (accusative, “12 days till the kalends of April”) is the 21st of March

Conversion Table:

DayJanuar - Ianuarius
1.Kalendis Ianuariis
2.a.d. IV Nonas Ianuarias
3.a.d. III Nonas Ianuarias
4.pridie Nonas Ianuarias
5.Nonis Ianuariis
6.a.d. VIII Idus Ianuarias
7.a.d. VII Idus Ianuarias
8.a.d. VI Idus Ianuarias
9.a.d. V Idus Ianuarias
10.a.d. IV Idus Ianuarias
11.a.d. III Idus Ianuarias
12.pridie Idus Ianuarias
13.Idibus Ianuariis
14.a.d. XIX Kalendas Februarias
15.a.d. XVIII Kalendas Februarias
16.a.d. XVII Kalendas Februarias
17.a.d. XVI Kalendas Februarias
18.a.d. XV Kalendas Februarias
19.a.d. XIV Kalendas Februarias
20.a.d. XIII Kalendas Februarias
21.a.d. XII Kalendas Februarias
22.a.d. XI Kalendas Februarias
23.a.d. X Kalendas Februarias
24.a.d. IX Kalendas Februarias
25.a.d. VIII Kalendas Februarias
26.a.d. VII Kalendas Februarias
27.a.d. VI Kalendas Februarias
28.a.d. V Kalendas Februarias
29.a.d. IV Kalendas Februarias
30.a.d. III Kalendas Februarias
31.pridie Kalendas Februarias
DayFebruar - Februarius
1.Kalendis Februariis
2.a.d. IV Nonas Februarias
3.a.d. III Nonas Februarias
4.pridie Nonas Februarias
5.Nonis Februariis
6.a.d. VIII Idus Februarias
7.a.d. VII Idus Februarias
8.a.d. VI Idus Februarias
9.a.d. V Idus Februarias
10.a.d. IV Idus Februarias
11.a.d. III Idus Februarias
12.pridie Idus Februarias
13.Idibus Februariis
14.a.d. XVI Kalendas Martias
15.a.d. XV Kalendas Martias
16.a.d. XIV Kalendas Martias
17.a.d. XIII Kalendas Martias
18.a.d. XII Kalendas Martias
19.a.d. XI Kalendas Martias
20.a.d. X Kalendas Martias
21.a.d. IX Kalendas Martias
22.a.d. VIII Kalendas Martias
23.a.d. VII Kalendas Martias
24.a.d. VI Kalendas Martias
25.a.d. V Kalendas Martias
26.a.d. IV Kalendas Martias
27.a.d. III Kalendas Martias
28.pridie Kalendas Martias
DayMarch - Martius
1.Kalendis Martiis
2.a.d. VI Nonas Martias
3.a.d. V Nonas Martias
4.a.d. IV Nonas Martias
5.a.d. III Nonas Martias
6.pridie Nonas Martias
7.Nonis Martiis
8.a.d. VIII Idus Martias
9.a.d. VII Idus Martias
10.a.d. VI Idus Martias
11.a.d. V Idus Martias
12.a.d. IV Idus Martias
13.a.d. III Idus Martias
14.pridie Idus Martias
15.Idibus Martiis
16.a.d. XVII Kalendas Apriles
17.a.d. XVI Kalendas Apriles
18.a.d. XV Kalendas Apriles
19.a.d. XIV Kalendas Apriles
20.a.d. XIII Kalendas Apriles
21.a.d. XII Kalendas Apriles
22.a.d. XI Kalendas Apriles
23.a.d. X Kalendas Apriles
24.a.d. IX Kalendas Apriles
25.a.d. VIII Kalendas Apriles
26.a.d. VII Kalendas Apriles
27.a.d. VI Kalendas Apriles
28.a.d. V Kalendas Apriles
29.a.d. IV Kalendas Apriles
30.a.d. III Kalendas Apriles
31.pridie Kalendas Apriles
DayApril - Aprilis
1.Kalendis Aprilibus
2.a.d. IV Nonas Apriles
3.a.d. III Nonas Apriles
4.pridie Nonas Apriles
5.Nonis Aprilibus
6.a.d. VIII Idus Apriles
7.a.d. VII Idus Apriles
8.a.d. VI Idus Apriles
9.a.d. V Idus Apriles
10.a.d. IV Idus Apriles
11.a.d. III Idus Apriles
12.pridie Idus Apriles
13.Idibus Aprilibus
14.a.d. XVIII Kalendas Maias
15.a.d. XVII Kalendas Maias
16.a.d. XVI Kalendas Maias
17.a.d. XV Kalendas Maias
18.a.d. XIV Kalendas Maias
19.a.d. XIII Kalendas Maias
20.a.d. XII Kalendas Maias
21.a.d. XI Kalendas Maias
22.a.d. X Kalendas Maias
23.a.d. IX Kalendas Maias
24.a.d. VIII Kalendas Maias
25.a.d. VII Kalendas Maias
26.a.d. VI Kalendas Maias
27.a.d. V Kalendas Maias
28.a.d. IV Kalendas Maias
29.a.d. III Kalendas Maias
30.pridie Kalendas Maias
DayMay - Maius
1.Kalendis Maiis
2.a.d. VI Nonas Maias
3.a.d. V Nonas Maias
4.a.d. IV Nonas Maias
5.a.d. III Nonas Maias
6.pridie Nonas Maias
7.Nonis Maiis
8.a.d. VIII Idus Maias
9.a.d. VII Idus Maias
10.a.d. VI Idus Maias
11.a.d. V Idus Maias
12.a.d. IV Idus Maias
13.a.d. III Idus Maias
14.pridie Idus Maias
15.Idibus Maiis
16.a.d. XVII Kalendas Iunias
17.a.d. XVI Kalendas Iunias
18.a.d. XV Kalendas Iunias
19.a.d. XIV Kalendas Iunias
20.a.d. XIII Kalendas Iunias
21.a.d. XII Kalendas Iunias
22.a.d. XI Kalendas Iunias
23.a.d. X Kalendas Iunias
24.a.d. IX Kalendas Iunias
25.a.d. VIII Kalendas Iunias
26.a.d. VII Kalendas Iunias
27.a.d. VI Kalendas Iunias
28.a.d. V Kalendas Iunias
29.a.d. IV Kalendas Iunias
30.a.d. III Kalendas Iunias
31.pridie Kalendas Iunias
DayJune - Iunius
1.Kalendis Iuniis
2.a.d. IV Nonas Iunias
3.a.d. III Nonas Iunias
4.pridie Nonas Iunias
5.Nonis Iuniis
6.a.d. VIII Idus Iunias
7.a.d. VII Idus Iunias
8.a.d. VI Idus Iunias
9.a.d. V Idus Iunias
10.a.d. IV Idus Iunias
11.a.d. III Idus Iunias
12.pridie Idus Iunias
13.Idibus Iuniis
14.a.d. XVIII Kalendas Iulias
15.a.d. XVII Kalendas Iulias
16.a.d. XVI Kalendas Iulias
17.a.d. XV Kalendas Iulias
18.a.d. XIV Kalendas Iulias
19.a.d. XIII Kalendas Iulias
20.a.d. XII Kalendas Iulias
21.a.d. XI Kalendas Iulias
22.a.d. X Kalendas Iulias
23.a.d. IX Kalendas Iulias
24.a.d. VIII Kalendas Iulias
25.a.d. VII Kalendas Iulias
26.a.d. VI Kalendas Iulias
27.a.d. V Kalendas Iulias
28.a.d. IV Kalendas Iulias
29.a.d. III Kalendas Iulias
30.pridie Kalendas Iulias
DayJuly - Iulius
1.Kalendis Iuliis
2.a.d. VI Nonas Iulias
3.a.d. V Nonas Iulias
4.a.d. IV Nonas Iulias
5.a.d. III Nonas Iulias
6.pridie Nonas Iulias
7.Nonis Iuliis
8.a.d. VIII Idus Iulias
9.a.d. VII Idus Iulias
10.a.d. VI Idus Iulias
11.a.d. V Idus Iulias
12.a.d. IV Idus Iulias
13.a.d. III Idus Iulias
14.pridie Idus Iulias
15.Idibus Iuliis
16.a.d. XVII Kalendas Augustas
17.a.d. XVI Kalendas Augustas
18.a.d. XV Kalendas Augustas
19.a.d. XIV Kalendas Augustas
20.a.d. XIII Kalendas Augustas
21.a.d. XII Kalendas Augustas
22.a.d. XI Kalendas Augustas
23.a.d. X Kalendas Augustas
24.a.d. IX Kalendas Augustas
25.a.d. VIII Kalendas Augustas
26.a.d. VII Kalendas Augustas
27.a.d. VI Kalendas Augustas
28.a.d. V Kalendas Augustas
29.a.d. IV Kalendas Augustas
30.a.d. III Kalendas Augustas
31.pridie Kalendas Augustas
DayAugust - Augustus
1.Kalendis Augustis
2.a.d. IV Nonas Augustas
3.a.d. III Nonas Augustas
4.pridie Nonas Augustas
5.Nonis Augustis
6.a.d. VIII Idus Augustas
7.a.d. VII Idus Augustas
8.a.d. VI Idus Augustas
9.a.d. V Idus Augustas
10.a.d. IV Idus Augustas
11.a.d. III Idus Augustas
12.pridie Idus Augustas
13.Idibus Augustis
14.a.d. XIX Kalendas Septembres
15.a.d. XVIII Kalendas Septembres
16.a.d. XVII Kalendas Septembres
17.a.d. XVI Kalendas Septembres
18.a.d. XV Kalendas Septembres
19.a.d. XIV Kalendas Septembres
20.a.d. XIII Kalendas Septembres
21.a.d. XII Kalendas Septembres
22.a.d. XI Kalendas Septembres
23.a.d. X Kalendas Septembres
24.a.d. IX Kalendas Septembres
25.a.d. VIII Kalendas Septembres
26.a.d. VII Kalendas Septembres
27.a.d. VI Kalendas Septembres
28.a.d. V Kalendas Septembres
29.a.d. IV Kalendas Septembres
30.a.d. III Kalendas Septembres
31.pridie Kalendas Septembres
DaySeptember - September
1.Kalendis Septembribus
2.a.d. IV Nonas Septembres
3.a.d. III Nonas Septembres
4.pridie Nonas Septembres
5.Nonis Septembribus
6.a.d. VIII Idus Septembres
7.a.d. VII Idus Septembres
8.a.d. VI Idus Septembres
9.a.d. V Idus Septembres
10.a.d. IV Idus Septembres
11.a.d. III Idus Septembres
12.pridie Idus Septembres
13.Idibus Septembribus
14.a.d. XVIII Kalendas Octobres
15.a.d. XVII Kalendas Octobres
16.a.d. XVI Kalendas Octobres
17.a.d. XV Kalendas Octobres
18.a.d. XIV Kalendas Octobres
19.a.d. XIII Kalendas Octobres
20.a.d. XII Kalendas Octobres
21.a.d. XI Kalendas Octobres
22.a.d. X Kalendas Octobres
23.a.d. IX Kalendas Octobres
24.a.d. VIII Kalendas Octobres
25.a.d. VII Kalendas Octobres
26.a.d. VI Kalendas Octobres
27.a.d. V Kalendas Octobres
28.a.d. IV Kalendas Octobres
29.a.d. III Kalendas Octobres
30.pridie Kalendas Octobres
DayOctober - October
1.Kalendis Octobribus
2.a.d. VI Nonas Octobres
3.a.d. V Nonas Octobres
4.a.d. IV Nonas Octobres
5.a.d. III Nonas Octobres
6.pridie Nonas Octobres
7.Nonis Octobribus
8.a.d. VIII Idus Octobres
9.a.d. VII Idus Octobres
10.a.d. VI Idus Octobres
11.a.d. V Idus Octobres
12.a.d. IV Idus Octobres
13.a.d. III Idus Octobres
14.pridie Idus Octobres
15.Idibus Octobribus
16.a.d. XVII Kalendas Novembres
17.a.d. XVI Kalendas Novembres
18.a.d. XV Kalendas Novembres
19.a.d. XIV Kalendas Novembres
20.a.d. XIII Kalendas Novembres
21.a.d. XII Kalendas Novembres
22.a.d. XI Kalendas Novembres
23.a.d. X Kalendas Novembres
24.a.d. IX Kalendas Novembres
25.a.d. VIII Kalendas Novembres
26.a.d. VII Kalendas Novembres
27.a.d. VI Kalendas Novembres
28.a.d. V Kalendas Novembres
29.a.d. IV Kalendas Novembres
30.a.d. III Kalendas Novembres
31.pridie Kalendas Novembres
DayNovember - November
1.Kalendis Novembribus
2.a.d. IV Nonas Novembres
3.a.d. III Nonas Novembres
4.pridie Nonas Novembres
5.Nonis Novembribus
6.a.d. VIII Idus Novembres
7.a.d. VII Idus Novembres
8.a.d. VI Idus Novembres
9.a.d. V Idus Novembres
10.a.d. IV Idus Novembres
11.a.d. III Idus Novembres
12.pridie Idus Novembres
13.Idibus Novembribus
14.a.d. XVIII Kalendas Decembres
15.a.d. XVII Kalendas Decembres
16.a.d. XVI Kalendas Decembres
17.a.d. XV Kalendas Decembres
18.a.d. XIV Kalendas Decembres
19.a.d. XIII Kalendas Decembres
20.a.d. XII Kalendas Decembres
21.a.d. XI Kalendas Decembres
22.a.d. X Kalendas Decembres
23.a.d. IX Kalendas Decembres
24.a.d. VIII Kalendas Decembres
25.a.d. VII Kalendas Decembres
26.a.d. VI Kalendas Decembres
27.a.d. V Kalendas Decembres
28.a.d. IV Kalendas Decembres
29.a.d. III Kalendas Decembres
30.pridie Kalendas Decembres
DayDecember - December
1.Kalendis Decembribus
2.a.d. IV Nonas Decembres
3.a.d. III Nonas Decembres
4.pridie Nonas Decembres
5.Nonis Decembribus
6.a.d. VIII Idus Decembres
7.a.d. VII Idus Decembres
8.a.d. VI Idus Decembres
9.a.d. V Idus Decembres
10.a.d. IV Idus Decembres
11.a.d. III Idus Decembres
12.pridie Idus Decembres
13.Idibus Decembribus
14.a.d. XIX Kalendas Ianuarias
15.a.d. XVIII Kalendas Ianuarias
16.a.d. XVII Kalendas Ianuarias
17.a.d. XVI Kalendas Ianuarias
18.a.d. XV Kalendas Ianuarias
19.a.d. XIV Kalendas Ianuarias
20.a.d. XIII Kalendas Ianuarias
21.a.d. XII Kalendas Ianuarias
22.a.d. XI Kalendas Ianuarias
23.a.d. X Kalendas Ianuarias
24.a.d. IX Kalendas Ianuarias
25.a.d. VIII Kalendas Ianuarias
26.a.d. VII Kalendas Ianuarias
27.a.d. VI Kalendas Ianuarias
28.a.d. V Kalendas Ianuarias
29.a.d. IV Kalendas Ianuarias
30.a.d. III Kalendas Ianuarias
31.pridie Kalendas Ianuarias

Leap year
DayFebruar - Februarius
24.a.d. VI Kalendas Martias (bis)
25.a.d. VI Kalendas Martias
26.a.d. V Kalendas Martias
27.a.d. IV Kalendas Martias
28.a.d. III Kalendas Martias
29.pridie Kalendas Martias

For the end of February, the table on the left shows the names of the days for a possible leap year. You can see that a.d. VI Kalendas Martias (six days before the kalends of March) occurred twice in successive years, and because of that, the designation for the following days did not have to change. Hence, the later term Dies Bissextilis, or “twice the sixth day” (before March). In the Middle Ages, this was used for the name of the entire leap year - annus bissextilis. Unfortunately, it is no longer possible to determine exactly whether this day was on the 24th or the 25th of February. That said, the 24th of February is more likely, as it was the date later used by the Church. Then again, in Roman law, the 25th of February was considered a leap day.

Later on in the Middle Ages, the classical form of denoting days was abandoned, as the names of days became normalised, and were considered nouns in their own right that did not need conjugation. For example, dates came to be written as Kalendas Iulii instead of Kalendis Iuliis.

The Roman calendar was originally divided into eight-day weeks. The individual days were marked with capital letters from A to H, while A was a day of rest. The Romans began to use the seven-day week at the beginning of the 3rd century AD, and its days were named after the gods to whom they were consecrated.

SundaySunSolis dies
MondayMoonLunae dies
TuesdayMarsMartis dies
WednesdayMercuryMercurii dies
ThursdayJupiterJovis dies
FridayVenusVeneris dies
SaturdaySaturniSaturni dies