Converting timestamp from local time to UTC

Is there a smooth way to convert a timestamp containing local time to UTC time?

The statement:

CONVERT TIME STAMP time_stamp TIME ZONE tz

INTO [DATE dat] [TIME tim]

[DAYLIGHT SAVING TIME dst].

Consideres time_tamp to be UTC time and then tries to convert it to the time zone specified in tz. What I would like to have done is the opposite, to have time_stamp represent the local time and then convert it back to UTC.

TIA!

/Armin