By default, if you do not configure the ntpconfig package, your cluster will be set up such that your nodes will be able to use the headnode as a time server. If you select a NTP server to sync up, please make sure that you do have a network connection to the server (whether it is on your LAN or on the Internet). The default, pool.ntp.org is a known good NTP servers pool provided by ntp.org.
NTP server entered below is for head node. In any case, cluster nodes will use headnode as NTP server.
If this is the first time for you to configure the ntpconfig package, hitting Cancel now will setup your headnode to act as a time server for your nodes.