Old Style and New Style dates

Old Style (O.S.) and New Style (N.S.) are sometimes used with dates to indicate whether the Julian year has been adjusted to start on 1 January (N.S.), even though documents written at the time use a different start of year (O.S.), or whether a date conforms to the Julian calendar (O.S.), formerly in use in many countries, rather than the Gregorian (N.S.). Closely related is double dating, which uses two consecutive years because of differences in the starting date of the year, or includes both the Julian and Gregorian dates.