Original: The notation '' Old Style '' ( OS ) is sometimes used to indicate a date in the Julian calendar , as opposed to '' New Style '' ( NS ) , which either represents the Julian date with the start of the year as 1 January or a full mapping onto the Gregorian calendar .
Simplified: People sometimes use the term Old Style or O.S. to refer to the Julian calendar , with N.S. or New Style referring to the Gregorian calendar .