Roman Dates in Julian calendar
The names of the months are: martivs, aprilis, maivs, ivnvs, ivlivs, avgvstvs, september, october, november, december, ianvarivs and febrvarivs; martivs is the first month of the Roman year
The number of days in each month is the same as in Western calendar, but the Roman calendar does have leap years in 200, 300, 500, 600, 700, 900, 1000, 1100, 1300, 1400, 1500, 1700, 1800, 1900, 2100, 2200 whereas the Gregorian calendar does not. As a result, the Julian (Roman) date is, during this century, 13 days behind the Gregorian date.
Example: September 19th in Gregorian corresponds to September 6th in Julian
The three most important days in each month are:
Kalendis - the first day
Nonis - the seventh day in martivs, maivs, ivlivs, and october and the fifth day in other months,
Idibvs - eight days after the Nonis, either on the thirteens or on the fifteenth day of the month.
All the other days are named after how many days they are before the next of these three, counting inclusively (that is, you count the day you start from).
Examples: August 9th in Julian is written as: ante diem V idvs avgvstvs
March 2nd is: ante diem VI nonas martivs
The day immediately before the Kalendis, Nonis or Idibvs is called pridie rather than given a number.
Example: March 31 is: pridie kalendas aprilis
In leap years (as given by the Julian calendar), the name ante diem VI kalendas martivs is given to both the 24th and the 25th of February.
The years are counted from the founding of Rome, or ab vrbe condita which occurred in 753 BC. The year 753 BC is therefore nvmbered I, 1 BC is DCCLIII (753) and 1 AD is DCCLIV (754), so you have to add 753 to convert Gregorian dates into Julian dates.
Examples: 2000 AD is MMDCCLIII (2753)
September 25, 1972 in Gregorian is pridie idvs september MMDCXXV in Jvlian
Febuary 2, 1854 is ante diem XII kalendas febrvarivs MMDCVII (observe that only 12 days are subtracted for the difference in leap years)