a number of people have suggested:
> hwclock --systohc

but to provide some variety, I'll mention that I learned to do:
clock -w -u (which seems to be a redhat-ism)
the portable version is:
hwclock -w -u

the -w is the same as --systohc, and the -u is for UTC (GMT).

if you're dual-booting between Windows and linux, tho; don't set your system
clock to GMT, as Windows can't handle offsetting that to your local time, so
you have to leave your system clock set to local time. :(

Carl Soderstrom
-- 
Network Engineer
Real-Time Enterprises
(952) 943-8700