No, not the double dating you did in high school when you didn’t have a date for the dance, this is the double dating that shows up in history books and genealogies. If you use a good genealogy data base like Family Tree Maker, your software may actually change or challenge any dates you put in pre- 1752 between January and March 24. Or you may have tried to figure out how to calculate a date during this time period, only to notice that you were off by three months somehow when you finally find the correct vital records. What is going on here?
The date 10/21 February 1750/51 is an example of double dating. It appears to have too many numbers, or it appears to be a guess to some readers, such as an approximate date. However, this is a real date on the calendar, along with an interesting story…
In 1752 there was a calendar change between the Julian and Gregorian calendar systems. The Julian Calendar had been invented during Roman times, and on the advice of his astronomer Julius Caesar started this new system in 45 B. C. It is officially known as the Old Style calendar. Under this calendar, New Year’s Day was on March 25th, and the last day of the year was March. This was considered the first month of the year.
Sometime in the medieval period, the astronomers noticed that the calendar year was not accurately measuring the solar year. In 1582, Pope Gregory XIII, reformed the calendar, which is called the New Style Calendar (Gregorian). It was first adopted in catholic countries and later by the Protestant countries. In order to make the adjustment, ten days were removed, so that 4 October 1582 was followed by 15 October 1582. England and the colonies adopted the new calendar in 1752, and removed eleven days from the calendar again, so on 2 September 1752 was followed by 14 September 1752.
Double dating was used in Colonial America for the dates between 1 January and 24 March on the years between 1582 and 1752. You will see this in old records, especially in civil records. Some church records used the old system, especially the Quakers, who used “First Month” for March, etc. In Quaker records “3rd Month” is used for May. For example the Quaker record 3/12/1719 will become 12 May 1719. This was to avoid using the Roman names January (the god Janus) or August (Augustus) which were pagan names.
If a date is given in double dating, it is correct to leave it as such, and not to try to calculate the date. Check to see which style was used in the original primary source. The date should be written 20 January 1745 OS (if it was Old Style) or as 20 January 1745/6.
"Most people find dates repulsive enough without encountering them disguised as fractions" by Historian Garrett Mattingly, from his book “The Defeat of the Spanish Armada” in which he opted just to use Gregorian dates (Wikipedia)
For more information:
An article from Ancestry’s Magazine Nov/Dec 2000, Volume 18, No. 6
“Marking Time: The Epic Quest to Invent the Perfect Calendar” by Duncan Steel, J. Wiley Publisher, 2000