FromDate[date] converts a date of the form y, m, d, h, m, s to an absolute number of seconds since the beginning of January 1, 1900.
FromDate converts between the forms returned by Date and AbsoluteTime.
FromDate assumes that both the date and the absolute time are to be given in the same time zone.
See The Mathematica Book: Section 2.13.3.
See also: ToDate.
Related package: Miscellaneous`Calendar`.