History of the Christian Calendar

The current Gregorian calendar is used in almost every country in the world. The Gregorian calendar was established in the year 1582 by Pope Gregory XIII. The Gregorian calendar follows the AD/BC era, which reckons years since the birth of Jesus Christ, which was placed in the year 754 Ab Urbe Condita.
== Roman Calendar ==
The Roman Calendar (Sometimes known as the Pre-Julian calendar) was the calendar before the Julian calendar. It consisted of 10 months and 304 days. In the Roman calendar, there were 10 months, instead than 12. The months were in sequence as: Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November and December. The final six months took the words five, six, seven, eight, nine, and ten. 
Several centuries later, a Roman ruler Numa Pompilius has modified the Roman calendar by adding 2 more last months to the year (Januarius and Februarius).
== Julian Calendar ==
The Julian calendar was founded in the year 45 BC. The calendar was named after Julius Ceasaer.