Old Style (O.S.) and New Style (N.S.) indicate a dating system from 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 the 20th century.
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 Lady Day (25 March) to 1 January (which Scotland had done from 1600), while the second discarded the Julian calendar in favour of the Gregorian calendar, removing 11 days from the calendar for September 1752 to do so.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.
In the Kingdom of Great Britain and its possessions, the Calendar (New Style) Act 1750 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".[5][a] (Scotland had already made this aspect of the changes, on 1 January 1600.[6][7]) The second (in effect[b]) adopted the Gregorian calendar in place of the Julian calendar. Thus "New Style" can either refer to the start of year adjustment, or to the adoption of the Gregorian calendar.
Below : Regnal Years (English Monarch's reigns)
1199-1216 : John
1216-1272 : Henry III
1272-1307 : Edward I
1307-1327 : Edward II
1327-1377 : Edward III
1377-1399 : Richard II
1399-1413 : Henry IV
1413-1422 : Henry V
1422-1461 : Henry VI
1461-1483 : Edward IV
1483 : Edward V (April-June)
1483-1485 : Richard III
1485-1509 : Henry VII
1509-1547 : Henry VIII
1547-1553 : Edward VI
1553-1554 : Mary (July-July)
1554-1558 : Philip & Mary
1558-1603 : Elizabeth I
1603-1625 : James I
1625-1649 : Charles I
1649-1660 : Commonweath Period
1660-1685 : Charles II (1st regnal year calculated from death of his father) Year 12 = 1660/1
1685-1688 : James II
1688-1694 : William & Mary
1694-1702 : William III
1702-1714 : Anne
1714-1727 : George I
1727-1760 : George II
1760-1820 : George III
1820-1830 : George IV
1830-1837 : William IV
1837-1901 : Victoria
1901-1910 : Edward VII
1910-1936 : George V
1936 Edward VII
1936-1952 : George VI
1952 Elizabeth II