I just spent an hour trying to figure out why the NTP service I installed wasn’t working, and then I came across this slightly important little fact:

Your internal workstation computers will not be able to use the server as a synchronisation source until the LOCAL(0) clock has stable time. This may take up to 15 minutes after starting the NTP daemon.

So thanks to Miles Brennan’s Linux Home Server HOWTO. Not sure why none of the other dozen tutorials I read failed to mention that, but it would have been nice…