The calendar used in the west today is the Gregorian calendar, also known as New Style (NS). However, this wasn’t always the case. The Gregorian calendar was adopted on different dates in different countries, for example, 1582 in most of Catholic Europe, but 1752 in Britain and its dominions; prior to this the Julian or Old Style (OS) calendar was in use.
The Gregorian Calendar was adopted to correct errors in the Julian Calendar, whereby the choice of leap years (compared to the earth’s journey around the sun), had caused the dates to slip back compared to the seasons. By the 1500s, this amounted to around 10 days. When countries adopted the Gregorian Calendar, they also skipped days to bring them back in line.
The varying dates of adoption in different countries, regions or city states, means that it isn’t enough to know a date of when an event occurred, but also the calendar in use at the time.
It is customary in genealogy, and best practice in history works generally, to record the calendar date as per the system in use at the time, and not make any adjustments for skipped days.
Pedigree Forge allows the entering of Old Style and New Style dates using the suffixes OS and NS as appropriate. For example:
1 Sep1600 OS
Would be interpreted as 1 September 1600, as per the Old Style (Julian) calendar, and is equivalent to 11 September 1600 in the Gregorian (New Style) Calendar.
1 Sep 1600 NS
Would be interpreted as 1 September 1600, as per the New Style (Gregorian) calendar, and is equivalent to 22 August 1600 in the Julian (Old Style) calendar.
When no specific calendar is specified, then Gregorian (NS) is assumed. It is possible to change this assumption from the “Dates” tab of the options dialog (from the menu bar: Tools > Options). This only changes the interpretation of the dates, no data is changed.
New Style and Old Style dates can also be combined with dual years, which indicate Modern Style, see below.
Technical Note 1: OS and NS dates are imported/exported to GEDCOM with the @#DGREGORIAN@ and @#DJULIAN@ escape sequences (GREGORIAN and JULIAN in version 7.0). If no calendar is given in the data, then no assumption is made on import/export—the date is left as was written.
Technical Note 2: It would be desirable to reject the “skipped” dates when the change was made from old style to new style calendars, but this is different depending on when the change was made and even knowing if the date is OS or NS doesn’t help—we also need to know where the event was recorded. For example, in Britain, 2 September 1752 was followed by 14 September 1752, the intermediate 11 dates not being valid, whereas in Spain, these dates did “exist”, the “missing” dates being 5 October to 14 October 1582 inclusive. Dates that don’t make sense are accepted (because they do exist in some places). Warnings are shown in “balloon tips”, and in validation reports, which are also combined with knowledge on where the event took place. General warnings are issued if the place can’t be determined, or if there is no knowledge of the convention in a particular place.