1. Roman Calendar (Numa Pompilius)
2. Roman Calendar (Decemviri)
3. Julian Calendar
4. Gregorian Calendar
* There have been many calendars throughout history. The Decemviri Calendar was introduced in 450BC. The Julian Calendar was introduced by Julius Caesar in 46BC. The Gregorian Calendar was introduced by Pope Gregory XIII in October 1582 but was not implemented by the British Empire until 1752. This entailed the removal of 11 days; Wednesday, September 2, 1752 was followed by Thursday, September 14, 1752. Such changes were necessitated by the rotation of the Earth. There are not actually 24 hours in a day but 23 hours 56 minutes 4.091 seconds. Likewise, a year is not 365 days but 365 days, 6 hours, 9 minutes, 10 seconds. This latter is why we have leap years.
The changeover from Julian to Gregorian can lead to confusion, and a duel dating system is often used to designate Old Style and New Style. Under the Julian Calendar, March 25 was the first day of the year.
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