How do I enable ntpd to serve ntp clients (using macOS 10.14 Mojave)?
From Macrumors (in the Mojave 'All the little things' thread):

Time sync: ntpd has been replaced by timed: not the old school unix one, but a new apple invention.

Have linked the timed Man page for ya:

https://www.unix.com/man-page/mojave/8/timed/