Old Style and New Style dates
In Britain and countries of the British Empire, Old Style or O.S. after a date means that the date is in the Julian calendar, in use in those countries until 1752; New Style or N.S. means that the date is in the Gregorian calendar, adopted on 14 September 1752 (New Style).
For example, William Shakespeare died on 23 April 1616 (Old Style). It is sometimes said that he died in the same day as Miguel de Cervantes, but the latter died ten days earlier, on 23 April 1616 (New Style).
A further complication is that for a long time the year in England began on 25 March (Lady Day), so for example Elizabeth I of England was recorded as having died on 24 March 1602 (Old Style); this would be written 24 March 1603 in the modern system of numbering years, which corresponds to 3 April 1603 (New Style). For complete avoidance of ambiguity, historians write dates in the ambiguous part of the year in slashed format, for example "24 March/3 April 1602/1603".
Catholic countries such as Italy, Poland, Spain, and Portugal were first to change to the Gregorian calendar. Thursday, 1582 October 4 was followed by Friday, 1582 October 15, with ten days "missing". Countries that did not change until the 1700s observed an additional leap year, necessitating 11 "missing days". Some countries did not change until the 1800s or 1900s, necessitating one or two more "missing days".
France changed from Julian to Gregorian Calendar on 9 December 1582 JU where the next day was 20 December 1582 GR. France had a special calendar in the period from 22 September 1792 GR to 31 December 1805 GR, the so-called French Republican Calendar.
In Russia, the terms "Old Style" and "New Style" have exactly the same meaning, the only difference being that Gregorian calendar have been introduced there much later — on February 14, 1918 (New Style). Hence the October Revolution of 1917 is so called, despite taking place on November 6 (New Style).