Dual dating

Dual dating is the practice, in historical materials, of indicating a date with what appear to be duplicate or excessive digits: these may be separated by a hyphen or a slash, or placed one above the other. The need for dual dating arose from the transition from an older calendar to a newer one. Another method used is to give the date of an event according to one calendar, followed in parentheses by the date of the same event in the other calendar, appending an indicator to each to specify which reference calendar applies.

As an example, in the date "10/21February 1750/51" – a style seen in the records of Great Britain and its possessions – the notation arises from the prospective or previous adoption of the Gregorian calendar and a concurrent calendar reform. (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 25March to 1January.)

After the Calendar (New Style) Act 1750 was passed, the notations "OS" (old style) and "NS" (new style) became dominant in historical writing about British and American events in the eighteenth century. This notation is also used when writing about Russian events in the early twentieth century.