If you only have irregular access to the Internet then ntpdate is useful. The package provides the ntpdate command to query an NTP server and to update your clock. You can do this manually as you see fit with:
$ ntpdate ntp.ktware.com
The advantage is that you will not have regular net transmissions as with the NTP server. This could save you up to 4MB per day. This could also be done as a cron job, for example.
#!/bin/sh #invoke ntpdate to set time from system clock if [ -x /usr/sbin/ntpdate ]; then /usr/sbin/ntpdate -s -t 5 ntp.ktware.com /sbin/hwclock --systohc fi
Make sure the script is executable:
# chmod a+rx /etc/ppp/ip-up.d/ntpdate
Then each time you connect the local clock will be synchronised with the NTP server.