Simples. From a Power Shell type (copy/paste) the following;

w32tm /config /manualpeerlist:pool.ntp.org /syncfromflags:MANUAL
Stop-Service w32time
Start-Service w32time

And that’s it. The pool.ntp.org is a round-robin of random selected NTP servers. As they say “This is usually good enough for end-users“. But you might want to add several NTP-servers yourself for redundancy?

Source: http://www.sysadminlab.net/windows/configuring-ntp-on-windows-server-2012

Then for the client machines you need to RUN as Administrator the following;

NET TIME /domain:mydomainname /SET /Y

And that’s it!