On Thomas Jefferson’s grave marker, his birth date is shown as “BORN APRIL 2, 1743 O.S.“
What is this “O.S.”?
Old Style – as in calendar…
The Old Style calendar, also known as the Julian calendar, was in used in England and the colonies until 1752, when the New Style, or Gregorian, calendar was adopted. This skipped eleven dates to bring the calendar year into step with the astronomical year, which had gotten out of sync due to not having leap days.. Thus, Jefferson’s birthday, April 2 under the Old Style calendar, is now celebrated on April 13, the New Style date. We still use the Gregorian or New Style calendar today.
We were 170 years late!
Pope Gregory XIII skipped ten days to to make the vernal equinox fall on 21 March, to synchronize calendar with the astronomical year. Countries that were Catholic were generally faster to adopt the Gregorian Calendar. The British called it the “New Style” calendar to achieve the same result without any reference to a Catholic Pope. They had to skip eleven days instead of ten days as they had fallen behind another day in the intervening 170 years.
Double Year Dates
Some dates before 1752 are expressed in this form: 1648/9. In Britain under the Julian calendar, the legal beginning of the year was Lady Day, March 25. Thus, dates from January 1 to March 25 before 1752 were often given both their Old Style and New Style years.
How Important?
For something that happened more than 250 years ago, an error of ten or eleven days is not going to make a difference. I am not going to stress about this unless I am trying to determine if something is an immediate direct reaction to another event around the time of the calendar change. Rarely will we have enough certainty and clarity to even consider such a small discrepancy.
It is good to be aware of the calendar changes, and that earlier times had this calendar “drift.” It’s good to understand that our astronomical year does not workout to exact while days. So to keep our solstices and equinoxes at about the same place on the calendar, it is good to have this Gregorian Calendar system.
All said, it does not have a huge impact on the stories we share.
It makes little difference to who we are or who our ancestors were.
More information
- “Gregorian Calendar.” In Wikipedia.
- FamilySearch Wiki. “Julian and Gregorian Calendars.”
- “Old Style (O.S.) | Thomas Jefferson’s Monticello.”