check_ntp_peer, which is included from plugin version 1.4.11, tests the quality of an NTP server. If you want to check the time deviation of a local server against an NTP server, you need to use the plugin
check_ntp_time, described in 7.7.1 Checking the system time via NTP in page 177.
Several parameters characterize the quality: the offset describes the time difference from other NTP servers (the reference servers). Jitter is a measurement of the fluctuations in the packet delay to a remote reference server, and stratum specifies the topological distance from the next atomic clock. Stratum 0 is the atomic clock itself, stratum 1 refers to an NTP server directly connected to an atomic ...