The two plugins
check_ntp_time compare the clock time of the local computer with that of an available NTP server in the network. If the Nagios server keeps time via NTP accurately enough, so that it can serve as a reference itself, then it can also be used as a network plugin, provided that the host to be checked in the network has an NTP daemon installed.
From plugin version 1.4.11, the plugins
check_ntp_peer (6.12 Health Check of an NTP Server with check_ntp_peer, page 154) replace
check_ntp, which contains the functions of both: the comparison of the local system time with an NTP server described here and the health check of the NTP server itself. ...