Dual dating

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 or a slash. This is also often referred to as double dating. The need for double 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 leap year correction of 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.

In English language histories and some contemporary documents, the terms "Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which calendar is/was being used for the given date . (For details see the article Old Style and New Style dates).

The Latin equivalents, which are used in many languages, are stili veteris (genitive) or stilo vetere (ablative), abbreviated st.v. and respectively meaning "(of) old style" and "(in) old style", and stili novi or stilo novo, abbreviated st.n. and meaning "(of/in) new style". The Latin abbreviations may be capitalised differently by different users, e.g., St.n. or St.N. for stili novi. There are equivalents for these terms in other languages as well, such as the German a.St. ("alten Stils" for O.S.).

"Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which date corresponds to an OS calendar that was used prior to a NS calendar. Eventually, the NS supplanted the OS, but the transition from the OS to the NS varied greatly by geography and the date of transition. Consequently, in places that have fully transitioned from an OS calendar to a NS calendar, dual dates appear in documents over an extended period of time, even centuries.

There is some confusion as to which calendar alteration OS or NS refers to: the change of the start of the year, or the transition of one style of calendar to another. Historically, OS referred only to the start of the year change to 1 January from March 25, and some historians still believe this is the best practice. However, OS and NS may refer to both alterations of the calendar.

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 an event in both the Julian calendar and the Gregorian calendar. Also, even before 1582, the year sometimes had to be double dated because different countries began the year on different dates.

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