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 always obtained by doubling (the "bissextum" (twice sixth) or bissextile day) until the late Middle Ages. The Gregorian calendar is proleptic before 1582 (assumed to exist before 1582). The following equation gives the number of days (actually, dates) that the Gregorian calendar is ahead of the Julian calendar, called the "secular difference" between the two calendars. A negative difference means the Julian calendar is ahead of the Gregorian calendar. where formula_2 is the secular difference and formula_3 is the year using astronomical year numbering, that is, use for BC years. formula_4 means that if the result of the division is