FromDate

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.

Examples

FromDate[{2000, 1, 1, 0, 0, 0}]
(* 3155673600 *)

FromDate[{1900, 1, 1}]
(* 0 *)

Please visit the official Wolfram Language Reference for more details.