The firmware already uses a pool of predefined NTP servers, all defined using hostnames:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/opensprinkler_server.cpp#L2285
The custom IP address is just for users who want to use their own NTP server (typically their router) to avoid accessing the Internet. It’s certainly possible to modify the firmware (and UI) to allow hostname for the custom NTP server too, but the predefined set of servers are typically sufficient for most users.