An NTP daemon is not likely to ever be added to m0n0wall. See section 1.2 of the Handbook.
http://doc.m0n0.ch/handbook-single/#id11553347Having said that, if you had to have this capability in m0n0wall it could probably be added in if you are willing to build your own custom image yourself, find someone to do this for you, or discover one already out there.
And there is always pfsense, a fork of m0n0wall that is quite extensible and already has an NTP daemon in its feature list.