Yo can use hwclock
hwclock --set --date=XXXXXXX (same as the date command)
After 24h or so you should set the date again with the same command and the correct time taken from a clock or the like.
You can also adjust it some times more to get an accurate idea of the systematic drift.
Once done with this you can insert the commands:
to adjust the time (at startup or periodically via cronjob) and copy the adjusted time to the system time.
Hope it helps, and take a look at man 8 hwclock
Nota, that you can get very accurate results using this command propperly.