I'd like to suggest an NTP (Network Time Protocol) monitor. Ideally it would:
– check stratum (warning/critical based on stratum – e.g. warning if not stratum 1 for a host with a GPS, critical if > 3)
– report the host with which the target is currently synchronising (and its offset)
– report things like jitter and stability (ppm)
Note that I don't think reporting the offset against the monitoring host (a la Nagios) is terribly useful – if I want to know the NTP performance of that machine I'd just use this monitor to target it directly. What I do want to know is the quality of the time available from the NTP servers.