Gregorian calendar

==Dual dating==
[[File:Memorial to John Etty (18373251064).jpg|thumb|Memorial plaque to John Etty in [[All Saints' Church, North Street, York]], recording his date of death as 28&nbsp;January&nbsp;{{sfrac|170|8|9}}]]
{{Main|Dual dating}}
During the period between 1582, when the first countries adopted the Gregorian calendar, and 1923, when the last European country adopted it, it was often necessary to indicate the date of some event in both the Julian calendar and in the Gregorian calendar, for example, "10/21 February 1750/51", where the dual year accounts for some countries already beginning their numbered year on 1 January while others were still using some other date. Even before 1582, the year sometimes had to be double-dated because of the different beginnings of the year in various countries. Woolley, writing in his biography of [[John Dee (mathematician)|John Dee]] (1527–1608/9), notes that immediately after 1582 English letter writers "customarily" used "two dates" on their letters, one OS and one NS.<ref>{{cite book| first1= Benjamin |last1= Woolley | title=The Queen's Conjurer: The science and magic of Dr. John Dee, adviser to Queen Elizabeth I |location= New York | publisher=Henry Holt | year= 2001 |page=173}}</ref>

===Old Style and New Style dates===
{{Main|Old Style and New Style dates|Calendar (New Style) Act 1750}} <!-- Per [[wp:FORK]], this section is a copy of the lead of that article. If change is needed, change the main article first then replicate it here. -->
"Old Style" (O.S.) and "New Style" (N.S.) indicate dating systems before and after a calendar change, respectively. Usually, this is the change from the [[Julian calendar]] to the Gregorian calendar as enacted in various European countries between 1582 and the early 20th century.

In [[England]], [[Wales]], [[Ireland]], and Britain's [[Thirteen Colonies|American colonies]], there were two calendar changes, both in 1752. The first adjusted the start of a new year from [[Lady Day]] (25 March) to 1 January (which [[Scotland]] had done from 1600), while the second discarded the Julian calendar in favour of the Gregorian calendar, removing 11 days from the September 1752 calendar to do so.{{sfn|Poole|1995 |pages=95–139}}<ref name="MS2">Spathaky, Mike [http://www.cree.name/genuki/dates.htm Old Style and New Style Dates and the change to the Gregorian Calendar] {{Webarchive|url=https://web.archive.org/web/20141011060050/http://www.cree.name/genuki/dates.htm |date=11 October 2014 }}. "Before 1752, parish registers, in addition to a new year heading after 24th March showing, for example '1733', had another heading at the end of the following December indicating '1733/4'. This showed where the Historical Year 1734 started even though the Civil Year 1733 continued until 24th March. ... We as historians have no excuse for creating ambiguity and must keep to the notation described above in one of its forms. It is no good writing simply 20th January 1745, for a reader is left wondering whether we have used the Civil or the Historical Year. The date should either be written 20th January 1745 OS (if indeed it was Old Style) or as 20th January 1745/6. The hyphen (1745-6) is best avoided as it can be interpreted as indicating a period of time."</ref> 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. Many [[Eastern Orthodox Church|Eastern Orthodox]] countries continue to use the older Julian calendar for religious purposes.
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