Old Style and New Style dates

Old Style (O.S.) and New Style (N.S.) indicate dating systems before and after a calendar change, respectively. Usually, they refer to the change from the Julian calendar to the Gregorian calendar as enacted in various European countries between 1582 and 1923.

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 25 March (Lady Day, the Feast of the Annunciation) to 1 January, a change which Scotland had made in 1600. The second discarded the Julian calendar in favour of the Gregorian calendar, skipping 11 days in the month of September 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.

For countries such as Russia where no start-of-year adjustment took place, O.S. and N.S. simply indicate the Julian and Gregorian dating systems respectively.

Britain and its colonies or possessions
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". (Scotland had already made this aspect of the changes, on 1 January 1600.) The second (in effect) adopted the Gregorian calendar in place of the Julian calendar. Thus "New Style" can refer to the start-of-year adjustment, to the adoption of the Gregorian calendar, or to the combination of the two.

Start-of-year adjustment
When recording British history, it is usual to quote the date as originally recorded at the time of the event, but with the year number adjusted to start on 1 January. The latter adjustment may be needed because the start of the civil calendar year had not always been 1 January and was altered at different times in different countries. From 1155 to 1752, the civil or legal year in England began on 25 March (Lady Day); so for example, the execution of Charles I was recorded at the time in Parliament as happening on 30 January 1648 (Old Style). In newer English-language texts, this date is usually shown as "30 January 1649" (New Style). The corresponding date in the Gregorian calendar is 9 February 1649, the date by which his contemporaries in some parts of continental Europe would have recorded his execution.

The O.S./N.S. designation is particularly relevant for dates which fall between the start of the "historical year" (1 January) and the legal start date, where different. This was 25 March in England, Wales, Ireland and the colonies until 1752, and until 1600 in Scotland.

In Britain, 1 January was celebrated as the New Year festival from as early as the 13th century, despite the recorded (civil) year not incrementing until 25 March, but the "year starting 25th March was called the Civil or Legal Year, although the phrase Old Style was more commonly used". To reduce misunderstandings about the date, it was normal even in semi-official documents such as parish registers to place a statutory new-year heading after 24 March (for example "1661") and another heading from the end of the following December, 1661/62, a form of dual dating to indicate that in the following twelve weeks or so, the year was 1661 Old Style but 1662 New Style. Some more modern sources, often more academic ones (e.g. the History of Parliament) also use the 1661/62 style for the period between 1 January and 24 March for years before the introduction of the New Style calendar in England.

Adoption of the Gregorian calendar
Through the enactment of the Calendar (New Style) Act 1750, the Kingdom of Great Britain, the Kingdom of Ireland and the British Empire (including much of what is now the eastern part of the United States and Canada) adopted the Gregorian calendar in 1752, by which time it was necessary to correct by 11 days. Wednesday, 2 September 1752, was followed by Thursday, 14 September 1752. Claims that rioters demanded "Give us our eleven days" grew out of a misinterpretation of a painting by William Hogarth.

Other countries
Beginning in October 1582, the Gregorian calendar replaced the Julian in Catholic countries. This change was implemented subsequently in Protestant and Eastern Orthodox countries, usually at much later dates and, in the latter case, only as the civil calendar. Consequently, when Old Style and New Style notation is encountered, the British adoption date is not necessarily intended. The start-of-the-year change and the calendar system change were not always adopted concurrently. Similarly, civil and religious adoption may not have happened at the same time or even at all. In the case of Eastern Europe, for example, all of those assumptions would be incorrect. Most nations of Europe and their colonies officially adopted 1 January as New Year's Day somewhat before they adopted the Gregorian calendar.

Bulgaria
Bulgaria officially adopted the Gregorian calendar on 1 April 1916, which became 14 April 1916. The Bulgarian Orthodox Church adopted the New Julian calendar on 20 December 1968.

France
France adopted the Gregorian calendar in December 1582. France changed New Year's Day to 1 January from 1564. Previously, France had counted various days as New Year's Day: Christmas, Easter-eve, 25 March (Lady Day).

Greece
Other countries in Eastern Orthodoxy eventually adopted Gregorian (or New Style) dating for their civil calendars, but most of these continue to use the Julian calendar for religious purposes. Greece was the last European country to change to Gregorian dating for civil purpose, in 1923. There is a 13-day difference between Old Style and New Style dates in modern Greek history.

Romania
Romania officially adopted the Gregorian calendar on 1 April 1919, which became 14 April 1919. This was commemorated in 2019 by the National Bank of Romania through the release of a commemorative coin of 10silver lei.

Russia
In Russia, Gregorian calendar dating came into use in early 1918, when 31 January 1918 was followed by 14 February 1918. (There had been a 13-day difference between Julian calendar and Gregorian calendar dates since 1 March 1900.)

It is common in English-language publications to use the familiar Old Style and/or New Style terms to discuss events and personalities in other countries, especially with reference to the Russian Empire and the very beginning of Soviet Russia. 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.

Sweden and Finland
In Sweden (and Finland, then part of Sweden) the Gregorian calendar was introduced with effect from 1March 1753. For practical purposes, the Julian calendar applied until 17February 1753. The difference between the two calendars was then 11 days, so the next day after 17February was designated 1March and thus the eleven days from 18 to 28February 1753 have never legally existed in Sweden.

In 1700, there was an attempt to gradually introduce the Gregorian calendar in Sweden. The plan was to disregard all leap days from 1700 to 1740 and thereby to reduce the difference between the calendars every four years. However, despite a good start in 1700, the Swedish authorities – due to the Great Northern War –  neglected to skip the leap days in 1704 and 1708. As a result, Sweden had its own calendar during that period, called the Swedish calendar and, according to this plan, it would not be until 1748 that the Swedish calendar would coincide with the Gregorian calendar. All of this caused Charles XII to decide, in 1711, that in February 1712 the country would return to the old calendar. This was done by inserting an extra leap day after the regular one that year: Friday, 30 February 1712, the so-called tillökningsdag (supernumerary day). It was to be another 41 years before Sweden attempted the task again.

When, as a result of the Treaty of Fredrikshamn, the Grand Duchy of Finland became part of the Russian Empire on 17 September 1809, Finland reverted to the Julian calendar in compliance with Russian norms. As a result, dates in certain contexts were often entered with both the Old and the New Style.

The Americas
The European colonies of the Americas adopted the New Style calendar when their mother countries did. In what is now the continental United States, the French and Spanish possessions did so about 130 years earlier than the British colonies. In practice, however, most surviving written records of what is now Canada and the United States are from the 20 mainland British colonies, where the Calendar (New Style) Act 1750 was applied, twenty-four years before the United States declared independence. Canadian records also include documents of New France that reflect the New Style, as France adopted it in 1582. The language used in the record is likely to be a good indicator of which calendar was being used for the dates given. The same logic applies to the Caribbean islands.

In Alaska, the change took place after the United States purchased Alaska from Russia. Friday, 6 October 1867, was followed by Friday, 18 October. Instead of 12 days, only 11 were skipped, and the day of the week was repeated on successive days, because at the same time the International Date Line was moved, from following Alaska's eastern border with Yukon to following its new western border, now with Russia.

Transposition of historical event dates and possible date conflicts
Usually, the mapping of New Style dates onto Old Style dates with a start-of-year adjustment works well with little confusion for events before the introduction of the Gregorian calendar. For example, the Battle of Agincourt is well known to have been fought on 25 October 1415, which is Saint Crispin's Day. However, 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 by using the Gregorian calendar. For example, the Battle of Blenheim is always given as 13 August 1704. However, confusion occurs when an event involves both. For example, William III of England arrived at Brixham in England on 5 November (Julian calendar), after he had set sail from the Netherlands on 11 November (Gregorian calendar) 1688.

The Battle of the Boyne in Ireland took place a few months later on 1 July 1690 (Julian calendar). That 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 (Julian). The latter battle was commemorated annually throughout the 18th 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. However, both events were combined in the late 18th century, and continue to be celebrated as "The Twelfth".

Because of the differences, British writers and their correspondents often employed two dates, a practice called dual dating, more or less automatically. Letters concerning diplomacy and international trade thus sometimes bore both Julian and Gregorian dates to prevent confusion. For example, Sir William Boswell wrote to Sir John Coke from The Hague a letter dated "12/22 Dec. 1635". In his biography of John Dee, The Queen's Conjurer, Benjamin Woolley surmises that because Dee fought unsuccessfully for England to embrace the 1583/84 date set for the change, "England remained outside the Gregorian system for a further 170 years, communications during that period customarily carrying two dates". In contrast, Thomas Jefferson, who lived while the British Isles and colonies converted to the Gregorian calendar, instructed that his tombstone bear his date of birth by using the Julian calendar (notated O.S. for Old Style) and his date of death by using the Gregorian calendar. At Jefferson's birth, the difference was eleven days between the Julian and Gregorian calendars and so his birthday of 2 April in the Julian calendar is 13 April in the Gregorian calendar. Similarly, George Washington is now officially reported as having been born on 22 February 1732, rather than on 11 February 1731/32 (Julian calendar). The philosopher Jeremy Bentham, born on 4 February 1747/8 (Julian calendar), in later life celebrated his birthday on 15 February.

There is some evidence that the calendar change was not easily accepted. Many British people continued to celebrate their holidays "Old Style" well into the 19th century, a practice that the author Karen Bellenir considered to reveal a deep emotional resistance to calendar reform.

Differences between Julian and Gregorian dates
The change arose from the realisation that the correct figure for the number of days in a year is not 365.25 (365 days 6 hours) as assumed by the Julian calendar but slightly less (c. 365.242 days). The Julian calendar therefore has too many leap years. The consequence was that the basis for the calculation of the date of Easter, as decided in the 4th century, had drifted from reality. The Gregorian calendar reform also dealt with the accumulated difference between these figures, between the years 325 and 1582, by skipping 10 days to set the ecclesiastical date of the equinox to be 21 March, the median date of its occurrence at the time of the First Council of Nicea in 325.

Countries that adopted the Gregorian calendar after 1699 needed to skip an additional day for each subsequent new century that the Julian calendar had added since then. When the British Empire did so in 1752, the gap had grown to eleven days; when Russia did so (as its civil calendar) in 1918, thirteen days needed to be skipped.

Other notations
The Latin equivalents, which are used in many languages, are, on the one hand, stili veteris (genitive) or stilo vetere (ablative), abbreviated st.v., and meaning "(of/in) old style"; and, on the other, stili novi or stilo novo, abbreviated st.n. and meaning "(of/in) new style". The Latin abbreviations may be capitalised differently by different users, e.g., St.n. or St.N. for stili novi. There are equivalents for these terms in other languages as well, such as the German a.St. ("alter Stil" for O.S.).