Source: http://defaultreasoning.wordpress.com/2009/11/16/synchronize-time-with-external-ntp-server-on-windows-server-2008-r2/

I can never seem to remember how to do this on 2008, so herewith instructions (copied from above link):

Time synchronization is an important aspect for all computers on the network. By default, the clients computers get their time from a Domain Controller and the Domain Controller gets his time from the domain’s PDC Operation Master. Therefore the PDC must synchronize his time from an external source. I usually use the servers listed at the NTP Pool Project website. Before you begin, don’t forget to open the default UDP 123 port (in- and outbound) on your firewall.

1. First, locate your PDC Server. Open the command prompt and type: C:\>netdom /query fsmo
2. Log in to your PDC Server and open the command prompt.
3. Stop the W32Time service: C:\>net stop w32time
4. Configure the external time sources, type: C:\> w32tm /config /syncfromflags:manual /manualpeerlist:”0.pool.ntp.org, 1.pool.ntp.org, 2.pool.ntp.org”
5. Make your PDC a reliable time source for the clients. Type: C:\>w32tm /config /reliable:yes
6. Start the w32time service: C:\>net start w32time
7. The windows time service should begin synchronizing the time. You can check the external NTP servers in the time configuration by typing: C:\>w32tm /query /configuration
8. Check the Event Viewer for any errors.

If you’re in South Africa, then pick one of the stratum 2 server on the following website:

http://www.time.org.za/

At the time of writing, they are:

Stratum-2 Servers