Some system clocks do not run all that accurately. Some hints on dealing with clock problems can be found in the Clock mini HOWTO at http://www.tldp.org/HOWTO/Clock.html, but the hwclock and adjtimex manual pages tend to be an easier read!
Be warned though, the Debian suggestion is not to fiddle with hwclock. It is highly recommended that you install the ntp-simple package (to run a daemon that checks regularly to keep your clock in sync) and/or ntpdate (to update your clock at each boot or to manually run a command to update your clock) to keep your clock accurate. These are discussed in more detail below.
If you install both ntp-simple and ntpdate and you manually set up cron to run ntpdate every hour then you will probably receive email every hour to tell you that ntpdate can't get the ntp socket (because the ntpd daemon from ntp-simple is probably using it!):
6 Oct 09:08:02 ntpdate: the NTP socket is in use, exiting
Just install one of them!