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.
But for the period between the first introduction of the Gregorian calendar on 15 October 1582 and its introduction in Britain on 14 September 1752, there can be considerable confusion between events in continental western Europe and in British domains.
Events in continental western Europe are usually reported in English language histories as happening under the Gregorian calendar.
Many British people continued to celebrate their holidays "Old Style" well into the 19th century, a practice that according to the author Karen Bellenir reveals a deep emotional resistance to calendar reform. For example, in the article "The October (November) Revolution" the Encyclopædia Britannica uses the format of "25 October (7 November, New Style)" to describe the date of the start of the revolution.
The need for change arose from the realisation that the correct figure for number of days in a year is not 365.25 (365 days 6 hours) as supposed by the Julian calendar but almost exactly 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year: the Julian calendar has too many leap years. When this usage is encountered, the reader should not assume that the British adoption date is intended, or that the 'start of year' change and the calendar system change were adopted concurrently, or even that religious adoption accompanied civil adoption.
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.
For example, the Battle of Blenheim is always given as 13 August 1704.
However confusion occurs when an event involves both.
In contrast, Thomas Jefferson, who lived during the time that the British Isles and colonies eventually converted to the Gregorian calendar, instructed that his tombstone bear his date of birth using the Julian calendar (notated O. for Old Style) and his date of death using the Gregorian calendar.
At Jefferson's birth the difference was eleven days between the Julian and Gregorian calendars; thus his birthday of 2 April in the Julian calendar is 13 April in the Gregorian calendar.
This maps to 11 July (Gregorian calendar), conveniently close to the Julian date of the subsequent [and more decisive] Battle of Aughrim on 12 July 1691.