Excel stores dates as sequential serial numbers so they can be used in calculations. By default, January 1, 1900 is serial number 1, and January 1, 2008 is serial number 39448 because it is 39,448 days after January 1, 1900. The fractional part of the number represents the time of day (e.g., .5 is 12:00 PM). Excel for Mac traditionally used the 1904 date system natively.