I ran into this question from a customer and Tam informed me that although the web GUI only accepts static IP's, it's possible to configure an NTP server in URL format via the CLI. The commands are as follows.
set sntp unicast-server domain-name time.google.com
The current web GUI doesn't support the entire functionality available via the CLI, but the cnMatrix team is adding more capability into the web GUI in every release. Supporting SNTP domain names is on that list for future support.