The first modern calendar was put into use in 45 B.C. by Julius Caesar. He decreed that there should be three years of 365 days each followed by one year of 366 days. This was to keep the calendar close to the actual time it took for the earth to make one trip around the sun. This was a tremendous improvement over the calendars that had been used up to that time. But it was still not totally accurate as it takes 365 days 5 hours 48 minutes and 47.8 seconds for the earth to completely revolve around the sun. Thus, those eleven minutes and 11.2 seconds make an appreciable difference over the course of several centuries. They make it appear to be a later day than the astronomical timekeeper says it should be.

In 325 A.D. the Nicean Council set the calendar for the Christian church. The council decreed that the first day of the year would be March 25. This was the day of the Feast of the Annunciation (commonly called Lady Day) which commemorates the visit of the Angel Gabriel to the Virgin Mary to inform her that she would be the mother of the Messiah. Note that March 25 is exactly nine months before Christmas, the day that birth is celebrated.

The correction of the calendar was made by Pope Gregory XIII in March of 1582. It was immediately adopted by most Roman Catholic countries. He first decreed that ten days should be eliminated from the current date. Thus, in the countries that adopted the new Gregorian Calendar, the day after October 4, 1582 became October 15, 1582. Additionally, the extra leap year day was not to be added on any year divisible by 100 but not divisible by 400. Thus the year 1600 was a leap year but the years 1700, 1800 and 1900 were not leap years. This made the calendar much more accurate -- off by less than one day for every 3000 years. One other change made with the adoption of this calendar was that January 1 would be the first day of the year.

The countries of Germany and the Netherlands converted to the Gregorian calendar in 1698.

The Gregorian Calendar was not adopted in Great Britain and the colonies of her empire until September of 1752. Because of this arose the custom of "double dating" for the days from January 1 through March 25 (the beginning of each new year). By choice of the record keepers, both of the years would be given, such as: March 1, 1700/1. This means it was the first day of March in the year 1700 using the Old Style (abbreviated to O.S.) and it was the first day of March in the year 1701 using the New Style.

To add to the confusion, in the case where, instead of month names the corresponding month numbers were used, April was considered to be the second month of the year and January was the eleventh month of the year. March was the first month although the new year did not start until the 25th. Thus, the first day of January in 1700/1 could be written as: 01-09-1700/1 or 01 (09) 1700/1. It is up to the reader of old records to realize this difference as the month numbers were usually not double-numbered as the years were.

In 1752 the British Empire finally adopted the Gregorian Calendar. But by this time their calendar was yet one more day off. So the day after September 2, 1752 was September 14, 1752. The use of the old style dating was dropped and January 1 became the new first day of each year. January was considered to be the first month of the year and no longer the ninth month. There was quite an uproar from the populus who believed that eleven days had been stolen from their lives.

The Netherlands had began using the Gregorian calendar in 1582. Thus, when the Dutch settled New Netherland, which eventually became the state of New York, all dates were recorded using the Gregorian calendar. The British colonies continued using the Julian calendar until 1752. However, the Quakers in Pennsylvania used the Julian calendar except they considered January 1 as the beginning of the new year.

Some countries did not adopt the Gregorian Calendar until later. These include: Japan in 1873, China in 1912, Russia in 1918, Greece in 1924 and Turkey in 1927.