To stay on time, you should really use ntpd. It is a daemon that runs all the time and makes tiny changes to the system clock. All you need to do is install ntp:
aptitude install ntp
after it's installed, I encourage you to go over to http://www.pool.ntp.org/zone/@ and pick a pool in your continental zone. Debian has some defaults, and they are fine, i just like using the ntp pool. If you want to change where you are syncing to, open up /etc/ntp.conf and change the "server" lines.
to manually sync Debian's time, just install "ntpdate"
apt-get install ntpdate
ntupdate-debian [a ntp server]