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Dual dating

From Wikipedia, the free encyclopedia

Dual dating is the practice, in historical materials, to indicate some dates with what appears to be duplicate, or excessive digits, sometimes separated by a hyphen, a slash or are placed one above the other. The need for dual dating arose from the transition from an older calendar to a newer one. For example, in "10/21 February 1750/51", the dual day of the month is due to the correction for excess leap years in the Julian calendar by the Gregorian calendar, and the dual year is due to some countries beginning their numbered year on 1 January while others were still using another date.

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.

Another method used is to give a date of an event according to one calendar, followed in parentheses by the date of the same event in the other, suffixing an indicator to each to specify which reference calendar applies.