had this problem with the time with wm5 & wm6. found this patch on htc site & now my time is alright after installing this patch. activesync/gprs/wifi settings were fine for me though.
from htc site:
Patch for Resolving Time Setting Loss Problem
After removing and then reinserting the battery in certain devices that run Windows Mobile 5 or earlier version, the time setting is lost and reverts back to the factory default setting. If you encountered this problem, please download this patch and install it on your device to fix the problem.
works on wm6 as well from my personal experience above