I find that xntp does not adjust the realtime hardware BIOS clock, only the
system clock.  Over time, if a significant skew builds up, on the next
reboot the clock will be thrown back.

If the master is rebooted, when there's a skew, all the slaves will get
their clocks reset.  If the slave is rebooted, several minutes will go by,
then the slave's clock will be jump by a large amount, when it is
resynchronized with a master.

Solution:  add the following to /etc/rc.d/init.d/xntpd, after the killproc
that stops the daemon:

   /sbin/hwclock --systohc

This will adjust the hardware clock with the accumulated skew, before the
system shutdown.