Synching to an External Time Source

Setting the Primary DC -  Windows Server 2003 to sync with an external time server:

Open the Registry Editor (regedt32.exe) make the following changes:

HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Parameters\Type

This registry entry determines which peers W32Time will accept synchronization from.
Change this REG_SZ value from NT5DS to NTP
This configures the PDC Emulator to synchronize from the list of reliable time servers specified in the NtpServer registry entry described below.

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

This registry entry controls whether the local computer is marked as a reliable time server (which is only possible if the previous registry entry is set to NTP as described above).
Change this REG_DWORD value from 10 to 5 here.

HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Parameters\NtpServer

This registry entry specifies a space-delimited list of NTP servers to synchronize with. The list may consist of one or more DNS names or IP addresses (if DNS names are used then 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 instead. 

See this link for a list of NIST time servers to choose from: http://tf.nist.gov/tf-cgi/servers.cgi

Now stop and restart the Windows Time service using the following commands:

net stop w32time
net start w32time

It may take an hour or so for the PDC Emulator to fully synchronize with the external time server because of the nature of the polling method W32Time uses. Depending on the latency of your Internet connection, the accuracy of the CMOS clock on your forest root PDC Emulator should be within a second or two of UTC.

Alternatively, if you don’t want to wait for time convergence to occur between your server and the external NTP server, you can run this command on your PDC:

w32tm /resync /rediscover