What happens if you boot to DOS (not a dos shell)?
Set your time there.....reboot back to DOS (off a write-protected diskette if possible)....if it's correct there, then you have a TSR and/or Virus messing with your clock.
Have you tried a virus scan with latest defs? Make sure it's set to scan EVERYTHING, with hieurstics at max (if option available).
Good Luck?