The calendar used in the west today is the Gregorian calendar, also known as New Style (NS). Before its adoption, the Julian or Old Style (OS) calendar was in use. The Gregorian calendar was adopted at different times in different countries — 1582 in most of Catholic Europe, but 1752 in Britain and its dominions.
The change corrected a drift in the Julian calendar caused by its leap-year rules, which by the 1500s amounted to about 10 days. Countries that adopted the Gregorian calendar also skipped days to bring dates back in line with the seasons.
It is customary in genealogy and historical writing to record dates as per the calendar system in use at the time, without adjusting for skipped days. Pedigree Forge supports this with OS and NS suffixes:
1 Sep 1600 OS — 1 September 1600 in the Julian (Old Style) calendar, equivalent to 11 September 1600 Gregorian.
1 Sep 1600 NS — 1 September 1600 in the Gregorian (New Style) calendar, equivalent to 22 August 1600 Julian.
When no suffix is given, Gregorian (NS) is assumed. This default can be changed on the Dates tab of Tools > Options.
OS and NS can be combined with dual years (see First day of the new year).
Note — OS and NS dates are imported and exported in GEDCOM using the @#DGREGORIAN@ and @#DJULIAN@ escape sequences (GREGORIAN and JULIAN in version 7.0). When no calendar is specified, no assumption is made during import or export.