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Correspondence with Our Calendar
 
The Romans called each year by the names of the Consuls in office, then as years of reign of the current Emperor. They also counted years from the mythical date of the foundation of Rome: ab urbe condita (A.U.C.). In 532, the monk Denys the Minorite arbitrarily set the birth of Christ on December 25, 753 A.U.C. and, therefore, the year 1 of the Christian era is the year 754 A.U.C. (There is no year 0). The year 1997 AD is the year 2750 A.U.C.
 
In 1582, Pope Gregory XIII decided to suppress 3 leap years over every 100 leap years to eliminate the small difference between the Julian year (see Julian Calendar) and the solar year. The difference between the Julian calendar and our (Gregorian) calendar is currently 13 days (our March 13, 1900 was exactly February 29 in the Julian calendar). This explains the 13 day difference between today date given according to the Julian calendar and the usual Gregorian date.