This article is about the 18th-century changes in calendar conventions used by Great Britain and its colonies, together with a brief explanation of usage of the term in other contexts. S.) are terms sometimes used with dates to indicate that the calendar convention used at the time described is different from that in use at the time the document was being written.

For a more general discussion of the equivalent transitions in other countries, see Adoption of the Gregorian calendar. There were two calendar changes in Great Britain and its colonies, which may sometimes complicate matters: the first change was to change the start of the year from Lady Day (25 March) to 1 January; the second was to discard the Julian Calendar in favour of the Gregorian Calendar.

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For example, William III of England arrived at Brixham in England on 5 November (Julian calendar), after setting sail from the Netherlands on 11 November (Gregorian calendar), in 1688.

The Battle of the Boyne in Ireland took place a few months later on 1 July 1690 "Old Style".

Usually, the mapping of new dates onto old dates with a start of year adjustment works well with little confusion for events which happened before the introduction of the Gregorian calendar.

For example, the Battle of Agincourt is universally known to have been fought on 25 October 1415, which is Saint Crispin's Day.

This maps to 11 July new style, conveniently close to the old style date of the subsequent [and more decisive] Battle of Aughrim on 12 July 1691 O. This latter battle was commemorated annually throughout the eighteenth century on 12 July, following the usual historical convention of commemorating events of that period within Great Britain and Ireland by mapping the Julian date directly onto the modern Gregorian calendar date (as happens for example with Guy Fawkes Night on 5 November).

The Battle of the Boyne was commemorated with smaller parades on 1 July.

Closely related is the custom of dual dating, where writers gave two consecutive years because of differences in the starting date of the year, or included both the Julian and Gregorian dates.

Beginning in 1582, the Gregorian calendar replaced the Julian in Roman Catholic countries.

To reduce misunderstandings about the date, it was normal in parish registers to place a new year heading after 24 March (for example "1661") and another heading at the end of the following December, "1661/62", to indicate that in the following few weeks the year was 1661 Old Style but 1662 New Style.