Not a too hard task, but there is “hard” to find the correct information. So here we go.
1) Edit or add a GPO and navigate to: Computer Configuration -> Administrative Templates -> System -> Windows Time Service -> Time Providers and open up “Configure Windows NTP Client”. Enable it and input the data you need. Probably pointing to your internal NTP-synced Domain Controller.
Click OK and open “Enable Windows NTP Client”. Enable that one as well and click OK.
After the next restart of the clients, the time will sync up again.
Also: I tried to configure the server with ntp-servers: ntp1.sp.se and ntp2.sp.se. Did never get any response to Server 2012. Changed the NTP-server to pool.ntp.org and things worked flawlessly after w32time reload and a w32time /resync