The configuration process is simple. Tick 'Obtain time from a Network Time Server' and fill out Primary NTP and Secondary NTP text boxes. If you don't know what to write there, go to www.ntp.org.
You can also opt IPCOP to act as time server for the rest of your network. Just tick 'Provide time to local network' and you're done. By synchronizing your IPCOP with NTP, your IPCOP clock will be very accurate and as a results, your logs can be used since they have precise time log.
There's manual synchronization method as well. See that 'Instant Update' command button? You can fill out required fields before that button manually and use it to confirm your changes.