If you’re researching American colonial-era documents, you need to understand that two different calendars were used in the colonies.
March 25 was the first day of the year until 1752.
Before 1752 the new year began on March 25. That is, the day after March 24th was March 25th in the following year. Because we are used to the idea that January 1 is the first day of a new year, that can be hard to get our heads around. For instance, January 1, 1700 came after December 31, 1700 rather than before it. April 1st of 1750 was only a month after March 1st of 1749, not more than a year later.
The bottom line is that we need to be alert to dates between January 1 and March 25. For instance, we might not otherwise understand how a will dated in November 1700 was proved at a court held in January 1700. Nor could we accurately translate dates recorded in the French or Spanish colonies of North America.
We use dual dating to distinguish between the old-style and new-style calendars. Nowadays we write dates like this: February 15, 1750/51. That helps us remember that the year was 1750 back then but 1751 in the modern calendar. (Contemporary scribes sometimes wrote dates in a similar way, and later records might occasionally specify dates as O.S. or N.S.)
American dates before 1752 were 11 days behind Europe.
The American and English calendar before 1752 was 11 days behind Europe — and 11 days behind the modern calendar. That’s why George Washington wouldn’t recognize his own birthday — he was born on February 11 by the calendar in effect at the time, but we celebrate his birth on February 22.
In 1752 this was fixed by skipping 11 days in the month of September. The day after 2 September 1952 was 14 September 1752.
The old-style Julian Calendar
The Julian Calendar was proposed by Julius Caesar, who decreed that it begin on January 1, 45BC. After some modifications by Augustus Caesar, the calendar of the Roman Empire looked a lot like the calendar we used today. It had twelve months and a year that was 365¼ days long, with a “leap day” added to February every four years.
The new-style Gregorian Calendar.
Caesar’s calendar had a problem — the solar year is actually only 365.2422 days, so that the Julian calendar “gained” a full day about every 128 years.
By the 1500s, the Julian calendar had gained ten days. and the calendar no longer matched the seasons. The discrepancy in the Julian Calendar was corrected by a calendar adopted by Pope Gregory in 1582. The Gregorian Calendar eliminated the extra February leap day in years divisible by 100, with the exception of years divisible by 400. Since the Julian Calendar was gaining about three days every 400 years, this scheme essentially eliminated “calendar creep”. The Gregorian calendar now gains a day only about every 3,032 years.
The switch from Julian to Gregorian.
The world moved from one calendar to the other in fits and starts after 1582. The Catholic countries of Europe and their colonies adopted the Gregorian Calendar almost immediately. The Protestant world delayed by a century or more, with Greece being the last European country to climb aboard in 1923. Non-Western nations didn’t adopt the universal modern calendar until about a century ago.
In late 1750 (early 1751 in the modern calendar) a bill was introduced in Parliament to have the calendar year begin on January 1 and to adopt the Gregorian calendar in the United Kingdom and its colonies. The result was that 1751 was a short year, running from March 25 to December 31. The 11-day adjustment took place in 1752 as mentioned above, so that by the end of 1752 Europe and the United Kingdom, and their overseas colonies, were using the same calendar.