Dual dating

On this marriage certificate, made out in 1907 in Warsaw (then part of the Russian Empire), the month is given as "November/December", and the day as "23/6". The Julian date 23 November corresponded to the Gregorian 6 December.

Excerpt of the cover page of a print of the Treaty of Lübeck (1629), with the Gregorian day (22) directly above the Julian (12), both before the name of the month, May. The treaty was concluded between Roman Catholic parties, who had adopted the Gregorian calendar, and Protestant parties, who had not.

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 "10/21 February 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 25 March to 1 January.)