My N800 is extremely accurate at all times, so I rarely do anything about the time - but there is a function which sets the hardware clock from the software/system clock, it works like this (in OS2007, I think, but ain't certain, that it's the same in OS2008):

/mnt/initfs/usr/bin/retutime --rtc-from-system

So, first set the time with rdate, then use the above command. It's the same as the 'hwclock' command on mainstream Linux, and also now Maemo 5.
EDIT: Clarify the above: On the N900 (Maemo5) the command is 'hwclock -w -u'. The 'retutime' command is for the old(er) systems.