Old Style and New Style dates

Old Style (O.S.) and New Style (N.S.) indicate dating systems before and after a calendar change, respectively. Usually, this is the change from the Julian calendar to the Gregorian calendar as enacted in various European countries between 1582 and 1923.

Issue 9198 of The London Gazette, covering the calendar change in Great Britain. The issue spans the changeover: the date heading reads: "From Tuesday September 1, O.S. to Saturday September 16, N.S. 1752".[1]

In England, Wales, Ireland and Britain's American colonies, there were two calendar changes, both in 1752. The first adjusted the start of a new year from 25 March (Lady Day, the Feast of the Annunciation) to 1 January, a change which Scotland had made in 1600. The second discarded the Julian calendar in favour of the Gregorian calendar, skipping 11 days in the month of September to do so.[2][3] To accommodate the two calendar changes, writers used dual dating to identify a given day by giving its date according to both styles of dating.

For countries such as Russia where no start of year adjustment took place, O.S. and N.S. simply indicate the Julian and Gregorian dating systems.

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