Roman Calendar Introduction

At the founding of the city of Rome the calendar had a 10 month calendar with 304 days per year. It underwent successive revisions until 5 AD when it stabilized around a structure that remained in use in some places into the 1900s.


The Roman world went through a series of revisions in its calendars between its first adoptions near the founding of the city of Rome and the present, with the current Gregorian calendar being the current calendar used throughout the world.

In each of the following sections we address the specific structure of the Roman calendar as it was used in the Roman world. We work backwards from the present in our presentation here, though, of course, each revision built on the earlier ones. Depending on your interest, you may not need to read all the way back through the account.

The Roman calendar had very unusual rules for counting days within months. As we present them here in this site we simply count days forward through each month. This was not how it was lived by people in the ancient Roman world. The pattern for day enumeration focused on the "Ides." For those doing specific historical work you need to consult a more complete source. I recommend a good encyclopedia.

Month names and abbreviations are also odd. We use English translations of what were originally Latin month names. Various sources in this area seem to smear the Latin and English month names together. It is hard to know what was actually the historical month name and what is a smeared or translated version.

New Years Day

This entire section uses the Roman, Pagan, rules for New Years day. The Christian Church, and the Christian world in general, moved away from the rules presented here and set New Years day on March 25. In all other respects the first Roman Calendar presented here is the same as the Julian calendar used in the Christian world.

Year Numbers

Years in this period were counted by those who lived them in two different ways. The first way was the year number from the first full year of the current emperor in Rome. This is seen, for example, in Luke were John the Baptist’s call is recorded in a certain year of the Emperor’s reign.

Years were also measured "AUC" which was a Latin abbreviation for an expression that meant "years from the founding of the city of Rome." This longer form kept the relationship between the various reigns in check. It also provides an audit for various uses.

As the world shifted to the Christian year numbering system, where the Christian year 1 was aligned with 754 AUC events recorded earlier in history were easily converted to dates "BC." Since all dates were written in roman numbers there was never any risk of including a year zero since this could not even be written, much less accidentally inserted. 1 BC was just another name for 753 AUC

Christian Year Number Roman Year Number
Some modern years...
2001 AD 2754 AUC
2000 AD 2753 AUC
1999 AD 2752 AUC
Some ancient years...
3 AD 756 AUC
2 AD 755 AUC
1 AD 754 AUC
1 BC 753 AUC
2 BC 752 AUC
3 BC 751 AUC
The first year AUC
753 BC 1 AUC


Early Church tradition put the year of Jesus’ birth in the year 753 AUC, but the Church also changed the definition of when years began so that it shifted to March 25 from January 1. As we explore in Jesus’ Life section, Jesus was probably born February 17, 754 AUC, in a month that would later be moved into 753 AUC by the change of New Years day to March 25.

The Last Roman Calendar -- Introduced 8 BC

The Roman calendar that lasts into the present as the Julian calendar was first introduced in its most modern form in the year 8 BC. It corrected for a bad leap day pattern from the previous calendar so the Roman world did not observe leap days following the conventional 4 year pattern until the first leap year in what we know of today as the year 8 AD. From 8 AD forward every year evenly divisible by 4 was a leap year, all other years were common.

This calendar reform was caused by Augustus Caesar who had the 8th month named after himself. Since he did not want a month with fewer than 31 days, and until the introduction of this calendar it only had 30 days, he changed the length of the 8th month from 30 to 31 days. The remaining months in the year were "rebalanced" so as to accommodate this change. The effect was that all the other months later in the year swapped lengths from 30 to 31 and so on. To make up for there now being 5 months of 31 days in this part of the annual calendar, February lost a day.

The following table shows month names in order along with conventional 3 letter abbreviations, the month lengths for common and leap years.

Number Name Abbr. Common Leap
1 January Jan 31 31
2 February Feb 28 29
3 March Mar 31 31
4 April Apr 30 30
5 May May 31 31
6 June Jun 30 30
7 July Jul 31 31
8 August Aug 31 31
9 September Sep 30 30
10 October Oct 31 31
11 November Nov 30 30
12 December Dec 31 31
Total: 365 366

Julius’ Roman Calendar, 45 BC to 9 BC

Julius Caesar instituted massive Roman calendar reform. These reforms impacted the year 46 BC, as we’ll see, and then established a stable calendar beginning in the year 45 BC. This calendar was used through the year 9 BC.

There was a "mistake" in the application of leap years and so every 3rd year on this calendar was a leap year instead of every 4th as it should have been to keep the calendar synchronized with solar phenomena. This caused 12 leap years to be observed before the last year this calendar was used in 9 BC. Only 9 such leap years should have happened, 3 extra. This is why the following calendar, introduced by Augustus in 8 BC skipped the first three leaps in what would have been 5 BC, 1 BC and 4 AD.

When this calendar was introduced the 7th month was called "Quintilis", though this name lasted for only 1 year. Beginning in the year 44 BC the 7th month was called Julis after Julius Caesar the man who introduced this calendar, a name it retains to this day.

Note the other unusual name in the calendar below. The 8th month, later renamed after Augustus in his later reforms, is here named "Sextilis."

This is probably the most "rational" of this family of calendars. It alternates in a clean pattern between months of length 31 days and 30 days, without exception. The leap month, February, is 30 days in its leap form and 29 in its common, short, form. Except for its occurrence mid year, the calendar is otherwise clean.

Number Name Abbr. Common
1 Januarius Jan 31 31
2 Februarius Feb 29 30
3 Martius Mar 31 31
4 Aprilis Apr 30 30
5 Maius Mai 31 31
6 Junius Jun 30 30
7 Julis Jul 31 31
8 Sextilis Sex 30 30
9 September Sep 31 31
10 October Oct 30 30
11 November Nov 31 31
12 December Dec 30 30
Totals: 365 366

Year of Confusion -- 46 BC

This year had a calendar unique in all Roman history. In this year Julius Caesar introduced calendar reforms that would begin on the following year. This year, though, was used to transition between the previous calendar and the new one to be started in 45 BC.

This year had 15 months. Three extra months were inserted into the calendar as shown. Since this calendar was only used for one year it has only one form, without a special leap form.

There appears to not have been month names assigned to the three intercalenary months.

Because of its unique shape it was known as the "year of confusion."

Number Name Abbr. Lengths
1 Januarius Jan 29
2 Februarius Feb 28
3 InterCal1[1] IC1 23
4 Martius Mar 31
5 Aprilis Apr 29
6 Maius Mai 31
7 Junius Jun 29
8 Quintilis Qui 31
9 Sextilis Sex 29
10 September Sep 29
11 October Oct 31
12 November Nov 29
13 InterCal2[1] IC2 34
14 InterCal3[1] IC3 33
15 December Dec 29
Total: 455

Roman Lunar Calendar 414 BC through 47 BC

This calendar was nominally a 355 day Lunar calendar. At this length it would be considered a pure lunar calendar, unlike the current luni-solar calendar used by Jews.

Some sources report that the calendar in this period eventually drifted to being a luni-solar calendar with the insertion of leap months every third year or so. They also indicate that this was a political process not one prescribed by arithmetic rules. It required an act of the Roman senate to insert such a month.

Because of this we have coded the online calendar system with a pure lunar implementation, but be aware that this period also included months of which we are unaware, and cannot count. This means that we are unable to insure day-accuracy for any date before the end of the year 47 BC.

This dramatic change in year lengths, without knowing exactly what the calendar corrections were, means that there is serious risk of drift error for any date passed down to us from this period.

Unusual features of this calendar include the unusual month lengths. A lunar calendar would be expected to alternate between 29 and 30 day month lengths. This calendar has 4 months of 31 days, 7 of 29 days and one of 28 days.

The following is the nominal calendar structure for this period.

Number Name Abbr. Lengths
1 Januarius Jan 29
2 Februarius Feb 28
3 Martius Mar 31
4 Aprilis Apr 29
5 Maius Mai 31
6 Junius Jun 29
7 Quintilis Qui 31
8 Sextilis Sex 29
9 September Sep 29
10 October Oct 31
11 November Nov 29
12 December Dec 29
Total: 355

Roman Calendar 713 to 415 BC

This calendar was a Lunar calendar with 12 months, much like we would expect in any Lunar calendar. It had a total of 355 days in each year. If the period 414 to 46 BC was actually a Luni-Solar calendar, then this period was probably a pure Lunar calendar. This would explain the transition at this point.

This calendar had new years day on March 1, not January 1 as we see in all later Roman calendars. This shifts the month order some and demonstrates the origin of the Roman calendar’s month names. In this calendar, for example, the month "December" actually is the 10th month, as the "Dec" prefix in the word December suggests.

This calendar introduced 2 months which had not previously been seen, pushing the calendar to its current 12 month form. January and February were the two new months and they were inserted at the end of the previous calendar which only had 10 months.

Before the introduction of this calendar the Roman world had a 10 month calendar with 304 days in each year which we will look at shortly.

The following is the shape of the calendar in this period of Roman history.

Number Name Abbr. Lengths
1 Martius Mar 31
2 Aprilis Apr 29
3 Maius Mai 31
4 Junius Jun 29
5 Quintilis Qui 31
6 Sextilis Sex 29
7 September Sep 29
8 October Oct 31
9 November Nov 29
10 December Dec 29
11 Januarius Jan 29
12 Februarius Feb 28
Total: 355

Roman Calendar used until 714 BC

This is the oldest known form of the Roman calendar. It had 10 months instead of the 12 seen in later calendars.

The following is the structure of this calendar.

Number Name Abbr. Lengths
1 Martius Mar 31
2 Aprilis Apr 30
3 Maius Mai 31
4 Junius Jun 30
5 Quintilis Qui 31
6 Sextilis Sex 20
7 September Sep 31
8 October Oct 30
9 November Nov 31
10 December Dec 29
Total: 304


This calendar has a significant feature for people following the Bible Time website story. This calendar had 304 days in each year. This was not Lunar nor Solar nor Luni-Solar. This calendar has a structure not aligned with any astronomical phenomena. This alone is interesting because it says its structure comes from some other source.

The Bible Calendar that we have already looked at had a periodicity of one Jubilee cycle, or 50 biblical years. In that period 18,240 days transpired.

This Roman calendar here has a related structure. With a strict periodicity of only one year, we miss an important feature of the 304 day calendar. 60 * 304 = 18,240 days also.

This is significant to the overall website story. This 10 month calendar is on average exactly ten twelfths the length of the Bible’s own intrinsic calendar. This is a smoking gun that suggests the Roman world, or at least those that settled the city of Rome had generated this calendar by dropping two months from the Bible’s own calendar. Why would they have done that? Because "they" had lost two of their tribes in a civil war some number of years before.

The Roman world also used a 15 year tax cycle. Four such cycles on this calendar would align with the Bible’s own Jubilee cycle exactly. Another smoking gun.

[1] The intercalenary months apparently had no names