Our modern calendar is the Gregorian calendar. Prior to it, the Julian calendar was used for over 16 centuries. For England and its colonies including America, Parliament passed the Chesterfield Act in 1750 which adopted the Gregorian calendar starting in 1752 which was already in use in most of western Europe.
To conform, it implemented the following changes:
- The new legal year began on January 1st rather than March 25th.
- Wednesday 2 September 1752 was followed by Thursday 14 September 1752–we lost 11 days!
As a result:
- 1751 was our shortest year of only 282 days (March 25, 1751 through December 31, 1751).
- 1752 was a short year of only 355 days (Wednesday, September 2 was followed by Thursday, September 14).
- The dates from 3 Sep 1752 through 13 Sep 1752 DO NOT EXIST!
In general, you should use reference dates in use at the time. These changes really only affect calculations of duration such as one’s exact age at a particular event like death.