`timed`: how to act as NTP server?

[I have two not-really-related questions about timed, so I'm asking them separately. The other one is "[timed not always working?](https://apple.stackexchange.com/questions/364892) ".] I just upgraded to High Sierra (yeah, I'm a bit behind the times), and I just discovered this new system time daemon, timed, which is evidently new with 10.13. Unlike my previous Macs, this machine does not seem to be acting as an NTP server -- that is, it is not accepting incoming NTP connections on port 123, even from the local host. I can see that it uses the configuration file /etc/ntp.conf, and presumably if I put the right lines in there it might do what I want, but I have no way of knowing whether it accepts all the same directives there as ntpd does, so I'm afraid it may be a bit of a crapshoot. Anybody know of an "official" or sanctioned way? Right now ntp.conf consists of the single line server time.apple.com. (At the moment I only care about accepting NTP connections from this machine, but eventually I may also want to accept them from the local network.) P.S. I was thinking there was a chance this might have something to do with firewall settings ("do/don't accept incoming connections"), although now I'm doubting that. At @nohillside's suggestion, I tried fetching time on it (hitting it on NTP port 123) from an outside machine, and that failed, too. (I've confirmed that I *can* fetch time by making NTP connections from this machine to other NTP servers, such as time.apple.com.)