TurnKey uses ntpd to update it's own time. The service name is 'ntp' (i.e. 'systemctl status ntp').

By default it shares it's time with the network. So you should be able to use your TurnKey Domain Controller as an NTP server if you wish. I just tested it (from a Debian desktop, getting time update from a local TurnKey server I have running) and it appears to work fine OOTB!? The only thing that I can think of off the top of my head is whether you have the firewall enabled or not? (It's configured by default, but not enabled). The default firewall config doesn't allow UDP 123 (the default NTP port) but as I note, unless you've enabled that, it shouldn't matter.

Even if you don't use your TurnKey server as an NTP server, so long as all your systems are getting their time from the same source, that should be ok. You'll just need to make sure that they're all using the same source. You can find (and change) the NTP sources that TurnKey uses within the NPT config file (/etc/ntp.conf). E.g. (the defaults from near the top of the config file):

# ntp servers
server 0.pool.ntp.org 
server 1.pool.ntp.org
server 2.pool.ntp.org
server 3.pool.ntp.org

If you change them, be sure to restart ntp:

systemctl restart ntp

However, you can tweak the NTP set up much more if you wish. I'm no expert on it, but the docs look fairly decent.