rdate [-p] [-s] [-u] [-l] [-t sec] [host…]选项
-p Print the time returned by the remote machine. -s Set the system time to the returned time. -u Use UDP instead of TCP as the transport. -l Use syslog to output errors (cron.warning) and output (cron.info). -t Set timeout in seconds for every attempt to retrieve date.使用 man rdate 可以查看