Gregorian calendar Gregorian equivalent of 29 February (Julian), to remember that 29 February is discounted. Thus if the calculated value is −4 the Gregorian equivalent of this date is 24 February. The year used in dates during the Roman Republic and the Roman Empire was the consular year, which began on the day when consuls first entered office—probably 1 May before 222 BC, 15 March from 222 BC and 1 January from 153 BC. The Julian calendar, which began in 45 BC, continued to use 1 January as the first day of the new year. Even though the year used for dates
Gregorian calendar the start of the year, and also aligned the British calendar with the Gregorian calendar, there is some confusion as to what these terms mean. They can indicate that the start of the Julian year has been adjusted to start on 1 January (NS) even though contemporary documents use a different start of year (OS); or to indicate that a date conforms to the Julian calendar (OS), formerly in use in many countries, rather than the Gregorian calendar (NS). Extending the Gregorian calendar backwards to dates preceding its official introduction produces a proleptic calendar, which should be used with some