Microsoft Excel day number is the number of days that have elapsed since the initial epoch defined as midnight preceeding December 31, 1899 in the Gregorian calendar. Microsoft Excel uses the Julian calendar leap year rule for 1900 (hence with leap day February 29, 1900) and the Gregorian calendar for the years 1901 - 9999; thus for dates from 1 March 1900 a time is stored as the number of days in the Gregorian calendar not from December 31, but from December 30, 1899, 00:00; optionally Microsoft Excel can also use the Apple Macintosh epoch, which avoids the complication by starting later; it represents times by a number that is 1462 less.