Dual dating

The need for dual dating arose from the transition from an older calendar to a newer one.

(The dual day number is due to the eleven days difference (at the time) between the Julian calendar date and the Gregorian one; the dual year is due to a change of start of year, from 25 March to 1 January.)

Long before the British Empire adopted the Gregorian calendar, the date of the start of the year caused difficulties.

It was to show this duality that the system of displaying two year numbers first came into use — examples may be seen on memorial tablets and in parish registers.

In 1752, England and its possessions changed the start of the year to 1 January, and also adopted the Gregorian calendar (on 2 September[c]).

Historically, OS referred only to the start of the year change, to 1 January from 25 March, and some historians still believe this is the best practice.

Although the OS/NS notation was originally used only to clarify the date of events in the British Empire, the usage was reprised in more recent English-language histories of Russia, which retained the Julian calendar until 1918.

Specifically, the months and days now correspond to those of the Gregorian calendar, but the year is expressed as an offset of the era.

[10] From 1 November 2011, writing old style dates with Chinese characters, never Arabic numerals, remains the standard in the PRC,[11][12] but new style dates may be written with either Arabic or Chinese numerals.

that both year numbers be entered into modern documents until a copy of the original primary source can be checked, verifying which style was used in the 'official record'.