[plt-scheme] how to get GMT time?

I'd appreciate it if anyone can tell me if this is the standard
way to get a GMT time from Mzscheme:

(define (gmt-date sec)
  (let ((offset (date-time-zone-offset (seconds->date sec))))
    (seconds->date (- sec offset))))

It seems ugly to have to call seconds->date twice.  Also,
is the offset always going to get me back to GMT, or do
I have to consider daylight savings time?

Thanks, --pg