The old Julian calendar began the new year on March 25. So, for example, 24 March 1451 was followed the next day by 25 March 1452. March was also considered the first month of the year. Old parish records commonly didn’t name the month but gave a month number. So, for example, a parish record of a baptism dated 16day 2mo: 1515 was 16 April 1515. It is very common to see conflicting dates off by 2 months as the result of people not understanding how the months were numbered.
The Gregorian calendar was introduced by Pope Gregory XIII in October 1582, and introduced two reforms:
- The first day of the new year was moved from March 25 to January 1.
- Ten days were dropped so that the Julian calendar day of Thursday, 4 October 1582 was followed by the first day of the Gregorian calendar, Friday, 15 October 1582.
This new calendar was primarily adopted by the catholic countries (France, Italy, Portugal, and Spain) in 1582, but not by the protestant countries, including the British Empire, until much later. England and her colonies switched to the new calendar in 1752.
Within England and her colonies, however, there was still a partial shift to the Gregorian calendar, and dates were sometimes recorded as if the year started on March 25 (Old style) and sometimes as if the year had started on January 1 (New style). Note this is NOT a shift to the Gregorian calendar as there was no 10 day shift in dates. So it was very common for parish records to begin the practice of double dating for dates between January 1 and March 25.
It also became a standardization for historians (and genealogists) to record the date “New style” as if the year began on January 1 even for records which occurred long before the Gregorian calendar was introduced. This was actually to avoid confusion, not create more.
As a Data doctor I commonly run into situations where on one child was born say September 1689, and the next child was born say January 1690. Obviously, this is impossible. The answer and the correction though is that the second child was born January 1690/1 and we properly record the date as January 1691. These sorts of confusions in the timeline would happen continuously throughout history if we did not standardize a year start date of January 1.
So in history or genealogy we use the following rules:
Rule 1: We use the day and month found on the document. This may seem like obvious common sense, however, shifting from one calendar to another can also cause a shift in the days - don't do it.
Rule 2: We assume the year started on January 1st. This does mean we often have to change the year found in a document.
Wikitree suggestion: Anytime a date is written Old style in a document (e.g.1715), but is recorded New Style in a date field (e.g. 1716), it is smart to use the double date in the biography to make this clear (e.g. 1715/6).