Old Style (O.S.) and New Style (N.S.) indicate dating systems before and after a calendar change, respectively. Usually, they refer to the change from the Julian calendar to the Gregorian calendar as enacted in various European countries between 1582 and 1923. Before as well as after the legal change, writers used the dual dating convention to specify a given day by giving its date according to both styles of dating (to ensure that the day concerned was identified unambiguously).
In England, Wales, Ireland, and Britain's American colonies, there were two calendar changes, both in 1752. The first of these was to adjust the start of a new year from 25 March (Lady Day, the Feast of the Annunciation) to 1 January, a change which Scotland had already made in 1600. The second discarded the Julian calendar in favour of the Gregorian calendar, skipping 11 days in the calendar for September 1752 to do so.
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 respectively.
Differences between Julian and Gregorian dates
The need to correct the calendar arose from the realisation that the correct figure for the number of days in a year is not 365.25 (365 days 6 hours) as assumed by the Julian calendar but slightly less (c. 365.242 days). The Julian calendar therefore has too many leap years. The consequence was that the basis for the calculation of the date of Easter, as decided in the 4th century, had drifted from reality. The Gregorian calendar reform also dealt with the accumulated difference between these figures, between the years 325 and 1582, by skipping 10 days to set the ecclesiastical date of the equinox to be 21 March, the median date of its occurrence at the time of the First Council of Nicea in 325.
Countries that adopted the Gregorian calendar after 1699 needed to skip an additional day for each subsequent new century that the Julian calendar had added since then. When the British Empire did so in 1752, the gap had grown to eleven days; when Russia did so (as its civil calendar) in 1918, thirteen days needed to be skipped.
Britain and its colonies or possessions
In the Kingdom of Great Britain and its possessions, the Calendar (New Style) Act 1750 (24 Geo. 2. c. 23) introduced two concurrent changes to the calendar. The first, which applied to England, Wales, Ireland and the British colonies, changed the start of the year from 25 March to 1 January, with effect from "the day after 31 December 1751". (Scotland had already made this aspect of the changes, with effect from 1 January 1600.) The second (in effect) adopted the Gregorian calendar in place of the Julian calendar. Thus "New Style" can refer to the start-of-year adjustment, to the adoption of the Gregorian calendar, or to the combination of the two. It was through their use in the 1750 act that the terms and notations "Old Style" and "New Style" came into common usage.
