Old Style or OS is a designation indicating that a date conforms to the Julian calendar, formerly in use in many countries, rather than the Gregorian calendar, currently in use in most countries. "Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which system is used in the British Empire and other countries that did not immediately change to the Gregorian calendar. In Britain it is usual to map most dates from the Julian year onto the Gregorian year without converting the day and month. However because the start of the Julian year was not always January 1 (see New Year's Day section in the Julian calendar article), OS/NS is particularly relevant for dates which fall between the start of the Gregorian year (January 1), and the start of the Julian year which was