It is hard to argue that logs are as good as correct they are. And correct timestamps of the logs are crucial to this. Internal clock is prone to drifting with time, in my experience I've seen some UTM appliances to drift as much as 40 minutes in just one year ! Even worse is that you can never be sure of the drift distribution over time - it may be incremental drift every day, or sudden jump due to who knows what.
To prevent this from happening I use NTP time synchronization on all of my servers/firewalls. If you have been in system administration for some time it is old news for you - just use ntpd daemon and pool.ntp.org servers located close to you, and you are set in 5 minutes.
In Checkpoint they took the hardening of the underlying OS to extreme and supplied only outdated ntpdate utility for the task, no ntpd for us. Not a big deal - I use the cron job below to run every 30 minutes ntpdate to update the firewall clock and so better be you. Cheers
30 * * * * /usr/sbin/ntpdate 1.uk.pool.ntp.org > dev/null