public time_t mktime ()
The values of other components of time are not restricted to their usual ranges. A negative value of GLib.Time.isdst causes mktime to attempt to determine if Daylight Saving Time was in effect.
If successful, recalculates and updates all fields in time to fit their proper ranges.
time since epoch as a time_t object on success or -1 if time cannot be represented as a time_t object.