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

Changing the value from NT5DS to NTP sets the server to sync from an NTP server on the internet.

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 ,0x9 to the end of each DNS name).  For example, to synchronize the PDC Emulator in your forest root domain with a NIST time server, change the value of the NtpServer registry entry from time.windows.com,0x9 to time.nist.gov,0x9. Alternatively, you can specify the IP address of this time server, which is 128.138.141.172.

HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Config\AnnounceFlags

Changing the value from 10 to 5 designates the server as a reliable time source.  In a correctly configured domain, client computers will update their system time from the domain controller.

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

You can check the current time status by running the following commands.

w32tm /query /status
w32tm /query /source