“Old Style” or “O.S.” after a date means that the date is in the Julian calendar. This was the calendar used in England and the colonies until 1752. “New Style” or “N.S.” means that the date is in the Gregorian calendar, which was adopted by England and the colonies on September 14, 1752.


To further complicate things, not all countries in the world switched over to the “New Style” or Gregorian calendar at the same time. For example, England and the colonies switched in 1752, but Spain and many other countries had already switched to “New Style” dating on October 4, 1582.


The “Old Style” follows the Julian manner of computing the months and days, or the calendar established by Julius Caesar, in which the year consists of 365 days and 6 hours. This ends up being a little more than 11 minutes too much, and in the course of time, between Caesar and Pope Gregory XIII, this surplus amounted to 11 days. Pope Gregory reformed the calendar by retrenching 11 days; this reformation was adopted by act of parliament in Great Britain in 1751, by which act eleven days in September, 1752 were retrenched, and the 3rd day was reckoned the 14th. This mode of reckoning is called “New Style.”


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