Julian vs. Gregorian CalendarThe calendar in common use today, known as the Gregorian calendar, was created in 1582 to replace the previously used Julian calendar. The Julian calendar, established in 46 B.C. by Julius Caesar, had 12 months, with three years of 365 days, followed by a fourth year of 366 days. The calendar was still just slightly longer than the solar year (by about 11 minutes per year), so by the time the year 1500 rolled around the calendar was 10 days out of synch with the sun.
To remedy the deficiencies in the Julian calendar, Pope Gregory XIII replaced the Julian calendar with the Gregorian calendar (named after himself) in 1582. The new Gregorian calendar dropped 10 days from the month of October for the first year only, to get back in sync with the solar cycle. It also retained the leap year every four years, except century years not divisible by 400 (to keep the accumulation problem from recurring). Of primary importance to genealogists, is that the Gregorian calendar was not adopted by many protestant countries until much later than 1592 (meaning they also had to drop a varying number of days to get back in synch). Great Britain and her colonies adopted the Gregorian, or "new style" calendar in 1752. Some countries, such as China, did not adopt the calendar until the 1900's. For each country in which you research, it is important to know on what date the Gregorian calendar took effect.
The distinction between the Julian and Gregorian calendar becomes important for genealogists in cases where a person was born while the Julian calendar was in effect and died after the Gregorian calendar was adopted. In such cases it is very important to record dates exactly as you found them, or to make a note when a date has been adjusted for the change in calendar. Some people choose to indicate both dates - known as "old style" and "new style."
Double DatingBefore the adoption of the Gregorian calendar, most countries celebrated the new year on March 25 (the date known as the Annuciation of Mary) to January 1. In most places, until the adoption of the Gregorian calendar, March 25 (the date known as the Annunciation of Mary) was the first day of the new year. The Gregorian calendar changed this date January 1 (a date associated with the Circumcision of Christ).
Because of this change in the start of the new year, some early records used a special dating technique, known as "double dating," to mark dates which fell between January 1 and March 25. A date such as 12 Feb 1746/7 would indicate the end of 1746 in the "old style" and the early part of 1747 in the "new style". When these dates are encountered, they should also be recorded exactly as found. If other evidence confirms the interpretation of the date, that should be indicated in a note.