You will be the first to have tried to build xntp (as far as I know), so I imagine you will have to do a little hacking. But that certainly is the right thing to be doing. Hurd/Mach currently has the primitive adjtime interface, but I suspect xntp can cope with that. Ultimately we should get fancy with the kernel's clock implementation; when someone is interested in that, we should probably just ask the xntp people want they like and do it that way.