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The development of the Roman calendar into
our modern Gregorian calendar

According to Livius , Numa Pompilius created the first Roman calendar.
This was a calendar based on the cycles of the moon. So every month counted 28 days and the year had 12 months:

Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November, December, Ianuarius en Februarius
Livius (1.19)

So the year in the Roman calendar started in Martius and during the republic the consuls were elected at the beginning of the year that started in springtime, also a good moment to raise an army and to start a campaign. Every 20 years an extra month had to be inserted: Mercedonius.
How this was done is unclear. Certain is that there was not a steady system and it all ended up in a mess.

Supposedly the decemviri in 451-450BC have placed Ianuarius and februarius at the beginning of the calendar

  Head stone
Keeping track of the years
The Roman did not really count the years, every now and then you can read in the old texts:"So and so many years after the founding of the city", but normally a specific year was indicated by the names of the two consuls.
A headstone (Rome 43):

To Coestus Herodianus' genius, taster of the divine Augustus later the same was administrator in Salustius' garden he died august 5 during the consulate of M.Cocceius and G. Vibius Rufinus


Exact year?
How do we  establish the exact year of a certain event or even better the exact date?
The problems here are:

With the help of events of which the date is secured scholars have converted the Roman calendar as good as possible to our modern Cornelia.

Exact date
Getting the date exactly right, is as you can imagine, even more difficult:
When a Roman author writes "September" in the Roman calendar, is that the same month that we call September in the Gregorian calendar?
The Romans had a strange way of pointing out the day of the month

The list of consuls gives the years as they are used today.

In what was essentially a farmers community it is of course very important to know when to seed and when to harvest. Some other peoples gave their months the name of the work to be done in that month.

Great improvements
In the time that Gaius Iulius Caesar was the pontifex maximus, he was, like all the other PMs  before him, responsible for maintaining the Roman calendar. He reformed the Roman calendar into the structure we still use, complete with a leap year every four years. To get the date right ( July back in the summer) he made the year 45BC 445 days long by inserting two extra months.

This Julian calendar was only 11 minutes/year wrong. It would take a long time before this gave new trouble. In 1582 Pope Gregorius (Gregory) XIII would refine this calendar by skipping the leap year every century (200, 300 etc), but keeping the leap year every 400th year (2000, 2400 etc.)
To get the date right from the start, he skipped 10 days in the year 1582
It would take a long time before all countries would use this calendar.


Hours in a day
As the romans did not have watches they used the sun to determine the time. They divided the day in twelve hours.
The actual length of the day played no role: an hour simply was 1/12 of that day. Noon was at the end of the 6th hour when the sun had reached it's zenith. With the sunset ended the twelfth hour. The Romans estimated the time with the help of the sun or used a sundial.


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