You may install a hardware clock, such as a radio or Global Positioning System (GPS) device. Configure the Windows Time service on the domain controller to synchronize with an external time server.
NOTE: Many GPS receivers and other time devices can function as NTP servers on a network. Can configure Active Directory synchronize time from these external hardware devices only if they are also acting as NTP servers on network.
If choose to implement an NTP time synchronization product other than the Windows Time service, you have to disable the Windows Time service on the domain reliable time source. All NTP servers need access to UDP port 123. The service is, W32tm.exe.
Remember, if you run DCPROMO, the Internet Time tab doesn't show in Date & Time DB. This is because Windows Server 2008 becomes a domain controller the default of obtaining the date & time via NTP from time.windows.com. By default, both Windows clients and servers will connect to time.windows.com and sync date and time using NTP.
The steps are:
1. From Command Prompt > type w32tm /stripchart /computer: time.windows.com /samples:5 /dataonly
2. Open UDP port 123 for outgoing
3. Open UDP port 123 for incoming
4. Now type following command to configure: w32tm /config /manualpeerlist: time.windows.com /syncfromflags:manual /reliable:yes /update
Remember, if you want to configure client computer for automatic domain time synchronization.
Now type the command > w32tm /config /syncfromflags: domhier /update
and again type, net stop w32time and net start w32time (both are separately)
or you can use Group Policy to configure the same!
For more info: http://support.microsoft.com/kb/816042
Hope it may help you!