Under KDE 4 there was warning message when no NTP service was installed/running if I remember correctly. This seems to be missing from Plasma 5. The expected behaviour would be to grey out the option to automatically sync time if NTP is not installed.
Thanks, but ensuring a working system is out of scope for system settings modules We don't have this available in timedated that we now base on: https://www.freedesktop.org/wiki/Software/systemd/timedated/