AbsoluteTime

AbsoluteTime[] gives the total number of seconds since the beginning of January 1, 1900, in your time zone.

AbsoluteTime[date] gives the absolute time specification corresponding to the given date specification.

Examples

Get the current absolute time:

AbsoluteTime[]
(* 3.94...*10^9 - a large number *)

Convert a date to absolute time:

AbsoluteTime[{2000, 1, 1}]
(* 3155673600 *)

Please visit the official Wolfram Language Reference for more details.