You could add ntp daemon to your startup scripts. From a root terminal:
rc-update add ntpd default
But if you don't want the ntp daemon running all the time, there are other ways to deal with it.
You might want to take a look at the file hwclock:
I wish I could tell you exactly how to set that, but it is still a bit of a mystery to me.
Mine is all hashed out, except for the first and last options, which I set to "local" in the first option, and left unchanged in the last option.
In my experience this clock setting business is not a simple matter.