Returns the serial number of the current date. The serial number is the date-time code used for date and time calculations. You can use the TODAY function only as a default value; you cannot use it in a calculated column.
Dates are stored as sequential serial numbers so they can be used in calculations. By default, December 31, 1899 is serial number 1, and January 1, 2008 is serial number 39448 because it is 39,448 days after January 1, 1900.