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 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.
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,[a] 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.
For instance, the calendar in the British Empire did not immediately change. Woolley, writing in his biography of John Dee (1527–1608/9), notes that immediately after 1582 English letter writers "customarily" used "two dates" on their letters, one OS and one NS. The Calendar Act 1750 altered the start of the year,[b] and also aligned the British calendar 11 days later to comply with the Gregorian calendar.
Japan, Korea, and China started using the Gregorian calendar on 1 January 1873, 1896, and 1912, respectively. They had used lunisolar calendars previously. None of them used the Julian calendar; the Old Style and New Style dates in these countries usually mean the older lunisolar dates and the newer Gregorian calendar dates respectively. In these countries, the old style calendars were similar, but not all the same. The Arabic numerals may be used for both calendar dates in modern Japanese and Korean languages, but not Chinese.
Japan started using the Gregorian calendar on 1 January 1873, locally known as "the first day of the first month of Meiji 6" (明治6年1月1日, Meiji rokunen ichigatsu tsuitachi). The preceding day, 31 December 1872, was "the second day of the twelfth month of Meiji 5" (明治5年12月2日, Meiji gonen jūnigatsu futsuka).
Japan currently employs two calendar systems: Gregorian and the Japanese era name calendar. Specifically, the months and days now correspond to those of the Gregorian calendar, but the year is expressed as an offset of the era. For example, the Gregorian year 2007 corresponds to Heisei 19. An era does not necessarily begin on January 1. For example, 7 January Shōwa 64—the day of the death of Emperor Shōwa—was followed by 8 January Heisei 1, which lasted until 31 December.
Korea started using the Gregorian calendar on 1 January 1896, which was the 17th day of the 11th lunar month in not only Korea, but also in China that still used the lunisolar calendar. The lunisolar Korean calendar is now used in very limited unofficial purposes only.
The Republic of China started using the Gregorian calendar on 1 January 1912, but the lunisolar Chinese calendar is still used along with the Gregorian calendar, especially when determining certain traditional holidays. The reference has been a longitude of 120°E since 1929, which is also used for Chinese Standard Time (UTC+8). China, Hong Kong, Macau, Malaysia, Indonesia, Singapore and Taiwan all have legal holidays based on the lunisolar Chinese calendar, with the most important one being the Chinese New Year.
To visually distinguish old and new style dates, writing new style dates with Arabic numerals but old style dates with Chinese characters, never Arabic numerals, is the standard in Chinese publications.
In Taiwan, even though new style dates are written in Chinese characters in very formal texts, it is now common to see Arabic numerals in new style dates in less formal texts. When writing old style dates, Chinese characters are usually used, but Arabic numerals may still be seen.[c] The calendar year in Taiwan is usually expressed as the "Year of the Republic" — counting Year 1 as the foundation of the Republic of China in 1912 CE.
There was a great deal of confusion when calendars changed, and the confusion continues today when evaluating historical sources. For instance, although the transition in the West often 'moved' the start of the year from March to January, and the nominal age of every person increased by 10 to 13 days by fiat when the Gregorian calendar replaced the Julian, the change of the calendar by official decree was actually often implemented on an historical date not in January, February, or March. As explained above, in the British Empire, including the American colonies, the OS change to NS was in September 1752. Thus, in addition to the 11-day discrepancy, the actual date of the OS/NS change in September 1752 created confusion in months other than January through March. For example, in the American colonies dual dating of the year other than in the months of January, February and March is extant in records in quite a few months. Further complicating accurate attempts at determining a date, in some cases historians did not differentiate between the years.
When 'translating' dates from historical documents to current documents for dates that have been incorrectly double dated by historians, both years should be entered into contemporary documents until a copy of the original primary source can be checked, verifying which style was used in the 'official record'. Often errors have been perpetuated from the early 19th century and still exist today. When 'translating' dates from historical documents to current documents for dates that have been correctly double dated by historians, the standard practice is to enter the earlier year first, and the later year second.
In either case, to avoid further confusion, contemporary researchers should be vigilant about annotating both dates with a notation indicating the type of date, and using a slash rather than a hyphen to indicate alternate dates.