Old Style and New Style dates

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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, it is sometimes remarked that William Shakespeare and Miguel de Cervantes died on the same date, 23 April 1616, but not on the same day. England was still using Old Style dating in 1616, while Spain was using New Style. Cervantes actually died ten days before Shakespeare.

A further complication is that the start of the Julian year was not always January 1st but was altered at different times in different countries. 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. Although this would correspond to 3 April 1603 if fully converted into the Gregorian calendar, the month and day of a British event are normally not converted. 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 used the French Republican Calendar from 22 September 1792 GR to 31 December 1805 GR.

Japan and China started using the Gregorian calendar in 1873 and 1912 respectively and used lunisolar calendars (similar, but not all the same in these countries) before. Neither of them used the Julian calendar. The Old Style and New Style dates in these countries mean the older lunisolar dates and the Gregorian calendar dates. The lunisolar Japanese calendar is no longer used, but the luni-solar Chinese calendar is still used with the Gregorian calendar in China and Taiwan.

In Russia, the terms "Old Style" and "New Style" have the same significance as elsewhere; however, the Gregorian calendar was introduced there much later—on February 14 1918 (New Style). Hence the October Revolution of 1917 is so called, despite having taken place on November 7 (New Style), i.e., October 25 (Old Style).

Occasionally using different calendars has caused confusion between contempories. For example one of the contributory factors for Napoleon's victory at the Battle of Austerlitz was the confusion between the Russians, who were using the Julian calendar, and the Austrians, who were using the Gregorian calendar, over the date that their forces should combine.

Usually in modern histories, to avoid confusion and to keep dates consistent, the OS dates are mapped onto NS dates with an adjustment for the start of the year to 1 January. For example modern histories all state that Charles I of England was executed on 30 January 1649. But Parliamentary documents investigating the regicide during the Restoration eleven years later all state that the event happened on 30 January 1648.<ref>House of Commons Journal Volume 8, 9 June, 1660 Regicides.</ref>

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 on 9 December 1582 (in France). For example the Battle of Agincourt is universally known to have been fought on 25 October 1415, which is Saint Crispin's Day. But for the period between the first introduction of the Gregorian calendar 4 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 ("New Style"). For example the Battle of Blenheim is always given as 13 August 1704. However confusion occurs when an event affects both. For example William III of England arrived at Brixham in England on 5 November, after setting sail from the Netherlands on 11 November! To add to the confusion, the Battle of Boyne which took place only a few months later in Ireland on 1 July "Old Style" is not mapped to 1 July "New Style" but is remembered as taking place on 12 July. The keeping of the recorded date (not a mapped date) for the anniversary of this battle has more to do with Protestants not at first recognising Gregorian dates, so they continued to celebrate the anniversary on their Protestant 1 July and now traditionally do so.