Mike wrote:

But when last checked GST and UTC differed by 16 hours, 22 minutes.

GST, Greenwich Sidereal Time, is an entirely different animal from
the old GMT, Greenwich Mean Time, and its modern successors, UTC &c.
The latter are based on measures of solar time (corresponding to the
position of the "mean sun"), while sidereal time is based on the
position of the stars. GST and UT are about the same at the autumnal
equinox each year, and are about twelve hours apart at the vernal
equinox; the difference between them changes by a little under four
minutes per day.

--
Odysseus