Converting dates in the calendar we use into Roman dates is tricky and involves some degree of compromise. The Roman calendar was altered many times as errors in previous calendars were corrected and political considerations led to compromises in those changes. So whether it is the day, the month or the year we convert into 'Roman' the final result may end up overall as something a Roman would not recognise. If you want to know something of the history of the calendar read on. If you just want a potted version and instructions on converting dates go to the conversion pages.

Many things about the Roman calendar are still the subject of dispute. The original sources for the information are few. This guide is based on the best evidence and modern scholarship but it may be wrong in some details. There is a list of sources at the end and reading them will give some idea as to the problems. However, what is presented here is a coherent and self-consistent version which is close to the truth.

The year
In the early days, Romans denoted years by the names of the two Consuls who ruled each year and that system continued long after other ways of denoting the year were used. Later they began to count the years from the foundation of the City of Rome. There is no single agreed date for that but a Roman writer Marcus Terentius Varro fixed the date as what we would call 753BC and that is the standard I shall use here. Romans used the letters AUC after these dates (in Latin ab urbe condita - from the foundation of the city).

Our own starting point for the calendar is no more certain. We count years from the supposed date of the birth of Jesus of Nazareth. The letters AD before a date stand for Anno Domini - the year of our lord. This phrase was first used by a monk called Dionysius Exiguus in the year 531. But Dionysius was wrong about the date of the birth of Jesus - scholars now put that three years earlier in 4BC not 1BC.

By the time of Dionysius, the Roman Empire was ending - in the West anyway - so any Roman expression of the date using a year dating from the birth of Jesus of Nazareth is not correct Roman form. Oddly, the most common use of Roman numerals today is to do just that - to give the year AD.

There is a move to replace the letters AD for designating the starting point for our calendar. The phrase which they stand for, 'year of our lord', might offend people from other religions whose Lord, if they have one, was born in a different year. AD can now be written CE short for Common Era. And BC - which stands from Before Christ - can be written BCE. According to Latin usage AD should come before the numerals such as AD353 whereas BC or the more modern CE or BCE being English follow the numerals.

The length of the year had been correctly determined by astronomers in different parts of the world many centuries before the Roman empire. But it was not until 46BC (708AUC) that the Roman leader Julius Caesar introduced a calendar reform to recognise that the year lasted almost exactly 365.25 days.

Originally Romans had ten months in the year of either 30 or 31 days. The winter period seems to have been without formal months and the year began in the Spring with March as winter ended and crops were planted. The ten month year is still recalled in the names of some of our months - September, October, November, and December come from Latin words for seven, eight, nine and ten.

Around 715BC the twelve month calendar was introduced, based on the phases of the Moon. It takes on average 29.5 days between one new moon and the next and so a twelve-month lunar year lasts 354 days but an extra day was added because even numbers were unlucky. The twelve months had between 28 and 31 days in each to make the year last 355 days. February was the shortest month with 28 days and every other year a whole extra month - called Mercedonius which alternated between 22 days and 23 days - was inserted after the 23rd day of February to try to keep the calendar in line with the solar year of approximately 365 days. At the end of Mercedonius the remaining five days of February were taken, so Mercedonius was followed by the 24th of February. But the arithmetic did not quite work - the system gives an average duration for the year of 366.25 days - and the calendar slowly drifted away from the seasons once more. Inserting an extra period to correct the calendar is called an intercalation.

The situation was made worse because the calendar was not a publicly available document. It was guarded by the priests whose job it was to make it work and determine the dates of religious holidays, festivals, and the days when business could and could not be conducted. Through both carelessness and abuse, the intercalations were not made even according to the flawed rules that had been laid down. By the time Gaius Julius Caesar took power in the mid 40s BC the calendar was in a mess and he decided to make a major reform. Indeed, he called in an Egyptian astronomer Sosigenes to advise him. As a result, the lunar year was abandoned in favour of a solar year lasting 365.25 days, and a total of 90 extra days were added to the year 46BC to bring the seasons back into line and set the spring equinox on the 25th day of March. Caesar also decreed that the year would in future start with January - although it had done so more many centuries, some parts of the empire were reverting to March for the start. And the 1st of January in the IV year of Caesar's consulship otherwise known as 709AUC or as we would call it 45BC was the start of the modern era of the Calendar.

However, there is some dispute about whether the months were put in their present form then or not. Modern scholarship suggests that the calendar introduced by Julius was in substantially its present form with February kept at 28 days (29 in a leap year) and that the other months were fixed at the lengths they now have. Earlier this century it was thought that January had 31 days and that short and long months alternated, making Sextilis (now called August), October and December short months of 30 days and September and November long months of 31. February was given 29 days to make a year of 365 days but every fourth year it had 30, thus adding the extra quarter day required to keep the calendar in tune with Earth's orbit round the Sun. But there is now evidence that this is wrong. One clue is that in a leap year, the extra day was added after the 23rd day of February, so the month in effect had two February 24ths. And the name for this in Roman times implies that February had 28 days. There is more on this later in the discussion on days.

Within two years of the start of these reforms Julius Caesar was dead, assassinated on the steps of the Senate in Rome on 15 March 44BC (710AUC). To honour him, the Senate decreed that the seventh month, called Quintilis, should be renamed Julius. But Caesar was gone before he could see how his reforms were working and before the first leap year (not a term the Romans used) was due in 41BC (713AUC). And perhaps that is why, with no-one to correct them, the priests or Pontifices who were supposed to keep track of the calendar misunderstood Caesar's decree and added the extra day to February every three years instead of every four. The Romans counted inclusively so to them every fourth year meant 1 2 3 4 5 6 7 8 9 10. As a result the first leap year was 42BC instead of 41 and they carried on with this error every three years until 9BC. It is strange that his instructions should have been misunderstood - he had been elected to the college of Pontifices himself two decades before his murder.

The mistake went unnoticed until around 9BC when Julius's successor - his great-nephew usually known now as Augustus Caesar and the first Roman Emperor - called for further changes. The Pontifices's error had gone unchecked for 36 years meaning that 12 extra days had been added instead of 9. To correct this Augustus ordered a halt in the leap years until the Earth had caught up with the calendar. In 8 BC the Senate also decided to honour the Emperor by renaming the month Sextilis as Augustus. In the past it was believed that the month lengths were then changed to their present form. Under this view, Sextilis had 30 days and the argument is that Augustus could not have fewer days than Julius so it was lengthened to 31. That would have left three 31-day months in succession as September was also believed to be 31 days long since Julius Caesar's reforms. So September and November were shortened to 30 days and October and December lengthened to 31. That left one too many days in the year so February was returned to its traditional length of 28 days to compensate. But nowadays this theory has fallen out of favour and it is though that the month lengths had already been set in their modern form by Julius Caesar so August was a 31 day month already and no further change was needed when Augustus was honoured. It was quite a small honour; after his death in AD14 the Senate pronounced him a God. But we can say with certainty that from 8BC there were the twelve familiar months with the same number of days they have now.


or as they are more correctly written as Romans did not have a 'J' or a 'U' in their alphabet


To get the calendar back on track, Augustus decreed that the first leap year after his reforms would be the year we call AD8 and they would occur every four years thereafter. And so they did. The calendar was to remain unchanged for more than one and half millennia and apart from a minor adjustment which began to be introduced in 1582, it remains substantially the same today.

While we count the days of the month forwards starting, for example, with the first of April and ending with the 30th, the Romans counted the days backwards. And not just from the end of the month, but from the first quarter and the middle of the month and then from the first day of the next month. So the day we would call the 20th day of January Romans would call the 13th day before the first of February. That arithmetic only works with inclusive counting - count every day from 20 January to 1 February inclusive.

This system harks back to the earlier Roman calendar based on the moon. The start of the month would be announced by the priest who saw the first faint sliver of light in the New Moon's black disc. He would call out - calare in Latin - and that is the origin of the Latin word for the first of the month the Kalendae or the Kalends. The next phase of the moon, the first quarter when it is exactly half a disc, were called the Nonae (usually Nones today) and the full moon was known as the Ides. The waning half was not marked. But even by the seventh century BC these divisions were formalised. The Kalendae were always the first day of the month. The Ides were fixed as the fifteenth day of a 31-day month or the thirteenth day of any other. The period leading up to the Ides was fixed as eight days and so the Nonae had to be on either the fifth day of a short month or the seventh day of a long month. At the time, only four months - Martius, Maius, Quintilis (Julius), and October had 31 days. The rest had 28 or 29. These fixed dates were continued by Julius Caesar when he converted the calendar, even though other months of 31 days were introduced. The new 'long' months of January, August and December still had the Nonae on the 5th day and the Ides on the 13th; that remained true after the changes introduced by Augustus.

Days were referred to as so many days before the Kalendae, the Nonae, or the Ides. So the 13th day of March was called

ante diem tertium idus Martias


before days third ides of March

or the third day before the Ides of March on the fifteenth. The Romans counted inclusively so they count 13, 14, 15 to get three days.

On the day itself they simply called it the ides of March or the kalends of May. And they referred to the day immediately before one of the three fixed dates as the pridie - the eve of. So the 14th of March was

pridie idus Martias

the eve of the ides of March.

On inscriptions these words were all abbreviated which means that we can ignore all the complex Latin endings to words which change with its grammatical position in the sentence. So we can simply write


And that meant to a Roman 2000 years ago the day we would call 13th of March.

There is one more little complexity. What to do about February? It has a variable number of days. Today we cope with a leap year by adding an extra day at the end of the month - February 29th. The Romans did it differently. Julius added the extra day after the 23rd day of the month - the same place that the bi-annual extra month Mercedonius had been inserted. So the leap day was like a very short Merecedonius and familiar to Romans of the time. But if you are naming days by counting backwards from the next event, adding an extra day affects the names of the previous days as well. In February the ides fell on 13th day of the month. The next day was 16 days before the 1st of March in a normal 28 day February. If an extra day was added, then the day after the ides of February would be 17 days before the 1st of March. But that did not happen. Because - like Mercedonius - the extra day was not part of February. The counting missed a beat, like this.

Roman Calendar for last ten days of February
Calendar for the last ten days of February
Not a leap year Leap year
Our date Roman date Our date Roman date
20 10 days before the start of March 20 10 days before the start of March
21 9 days before the start of March 21 9 days before the start of March
22 8 days before the start of March 22 8 days before the start of March
23 7 days before the start of March 23 7 days before the start of March
24 6 days before the start of March 24 6 days before the start of March
25 5 days before the start of March 25 again 6 days before the start of March
26 4 days before the start of March 26 5 days before the start of March
27 3 days before the start of March 27 4 days before the start of March
28 eve of the start of March 28 3 days before the start of March
29 eve of the start of March

In other words the extra day did not alter the calendar until after the 24th day - a time when it was always hard to tell the date in Rome. Because the 24th was normally called the sexto kalendae the extra day was called the bis-sexto-kalendae meaning the double, twice or again sixth day before the start of March. Sometimes a Leap Year is referred to as a bissextile for that reason. This odd way of doing things has led some scholars into saying that the extra day was added after the 24th, when in fact logic - and history - determines that it was added after the 23rd. This name and system also implies that February did indeed have 28 and 29 days after Julius Caesar's reforms.

The week
We do not need to know about weeks to deal with dates. Calendar reform of months or years has not normally affected the cycle of days of the week. Weeks are a purely human rhythm, whereas the calendar attempts to reflect and reconcile three separate natural rhythms of the Universe outside - the Earth's day, the Moon's month, and the Sun's year.

The Romans used an eight-day cycle for their civil organisation with a market or Nundinae every eight days. This internundium was indicated on calendars by the letters A to H. And there were complicated rules about what could or could not be done on certain days. The Fasti were days when the banks and courts were open and business could be done. But the Nundinae were market days - Nefasti - when the courts were closed. Other days were set aside for religious festivals and some days were designated when public meetings could be held.

The seven day week with Sunday as its Holy Day was introduced by the Roman emperor Constantine as part of the Christian reforms in AD321.

This page has been translated into Romanian by Alexander Ovsov of Web Geek Science.


  1. E G Richards Mapping Time - The Calendar and its History Oxford University Press, Oxford 1998
    Excellent and thorough guide by a man who seems to have read all the sources and made sense of them. Also contains information on converting calendars and the computer algorithms to do so. Concludes with the bibliography to end all bibliographies.

  2. David Ewing Duncan The Calendar Fourth Estate, London 1998
    A popular and readable new guide to the history of the calendar. A bit un-detailed at times.

  3. O A W Dilke Mathematics and Measurement British Museum Press, London 1987
    Brief but scholarly work on all aspects of measurement by ancient civilisations including time and the calendar.

  4. Frank Parise The Book of Calendars Facts on File, New York 1982
    Lots of tables and explanations of them.

  5. E J Bickerman Chronology of the Ancient World Thames and Hudson, London 1968
    Very useful reference material, chronologies of events and tables of conversions.

  6. Agnes K Michels The Calendar of the Roman Republic Princeton 1967
    The Roman calendar before Julius Caesar's reforms.

  7. Ovid Fasti translated by J G Frazer, revised G P Gould, Harvard University Press (Loeb's Classical Library) 1996.
    The real thing but hard to find the useful information.

  8. Encyclopaedia Britannica
    fairly comprehensive information on the Calendar and its reform. Different editions take different approaches.

All material on these pages is Paul Lewis 1999-2012