These instructions are taken from windowsnetworking.com. More details and options can be found on their site.
Edit the following registry entries:
HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Parameters\Type
Change the value from NT5DS to NTP. This sets the server to sync from an NTP server on the internet.
HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Config\AnnounceFlags
Change the value from 10 to 5. This marks the server as a reliable time server. In a correctly configured domain, client computers will update their system time from the domain controller.
HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Parameters\NtpServer
This registry entry specifies a space-delimited list of stratum 1 time servers from which the local computer can obtain reliable time stamps. The list may consist of one or more DNS names or IP addresses (if DNS names are used then you must append ,0x1 to the end of each DNS name). For example, to synchronize the PDC Emulator in your forest root domain with tock.usno.navy.mil, an open-access SNTP time server run by the United States Naval Observatory, change the value of the NtpServer registry entry from time.windows.com,0x1 to tock.usno.navy.mil,0x1 here. Alternatively, you can specify the IP address of this time server, which is 192.5.41.209.
Now you can either reboot the server or stop and start the time service by running the following two commands:
net stop w32time
net start w32time