Here are the main steps to configure Windows time on Domain Controllers using an external time source and also main operation tasks to do: the main technet reference is:
http://technet.microsoft.com/en-us/library/cc773061(v=ws.10).aspx

and http://technet.microsoft.com/en-us/library/cc784800(v=ws.10).aspx

External Time source: http://support.ntp.org/bin/view/Main/WebHome  and

http://support.ntp.org/bin/view/Servers/NTPPoolServers

1) To configure the Windows time service on the Forest Root PDC Emulator, perform the following actions:

Ensure outgoing and incoming 123/udp ports are enabled between the PDC Emulator and the external time source: ex: 193.26.35.12 or 0.europe.pool.ntp.org …

First check the time difference between the PDC Emulator and the time source by running the following;

        w32tm /stripchart /computer:193.26.35.12 /samples:5 /dataonly

Loosely synchronise the time on the PDC emulator to match the returned information from the external time source. Enter the following command;

        w32tm /config /manualpeerlist:193.26.35.12 /syncfromflags:manual /reliable:yes /update

If the PDC Emulator FSMO role is moved to another DC, perform the following actions.

              Run the following command;
                                        w32tm /config /syncfromflags:domhier /reliable:no /update
              Then
                Net stop W32Time
                Net Start W32Time

Other DCs can be configured as reliable time sources, to do this first perform the tasks where used to configure the PDC Emulator. Following this enter ;

           w32tm /config /syncfromflags:domhier /reliable:yes /update
           w32tm /config /reliable:yes /update

Set the /reliable flag to either yes or no as desired.

Member Servers:

It is possible to configure a member server to synchronise its system time with an external source.   To configure in this way follow the steps outlined above.   The preferred method with the AD environment is to synchronise automatically using the Domain hierarchy as the source.
To configure in this way follow these steps;

      w32tm /config /syncfromflags:domhier /update
      Net stop W32Time
      Net start W32Time

Restoring Windows Time Settings Back to Defaults:

If there are problems with the Windows Time service and you wish to reset the service back to it’s default settings, follow these steps;

    Net stop W32Time
    w32tm /unregister
    w32tm /register
    Net start W32Time

Monitoring Windows Time Settings and Resync:

w32tm /monitor

w32tm /resync /rediscover         ; to reforce synchro

Dump the Windows Time configuration:

w32tm /query [/computer:<target>] {/source | /configuration | /peers | /status} [/verbose]

Display the current time zone settings:

w32tm /tz

2) Interesting Web resources:

http://msmvps.com/blogs/acefekay/archive/2009/09/18/configuring-the-windows-time-service-for-windows-server.aspx

http://binarynature.blogspot.fr/2012/04/configure-active-directory.html#!/2012/04/configure-active-directory.html
 
http://social.technet.microsoft.com/Forums/en-US/winservergen/thread/e6295b20-9821-4882-bc62-931a3c5fdd43/
 
http://windowshell.wordpress.com/2012/01/02/configuring-an-authoritative-time-source-for-your-windows-domain/