time service in group policy

This should start you off in the right direction

http://blogs.msdn.com/w32time/pages/group-policy-settings-explained.aspx

where do i find this setting inside the gpo?

can you explain the steps to do that?and i need also that my dc will replicate the time between them

what should be the configuration of ntp server parameter in the dc that is not  the pdc?

I am going to go off track here and see if this is a solution that may interest you. It does not use group policy and works fine in our environment. We have the DC that holds the PDC Emulator FSMO role pointing to an external time server as per.

http://support.microsoft.com/kb/816042

(Your 2008 DC's should be sharing the FSMO roles)

For the other DC's, Servers, Workstations etc we configured  the W32Time service to use its default settings by doing the following

- Stop the w32time service
- Running "w32tm /unregister && w32tm /register/"at the command prompt (without quotes)
- Starting the w32time service

Your other DC will get its time from the PDC emulator and your workstations from the DC that authenticated them.

All Servers and workstations should now be in perfect sinc without the need for group policy

if i understand you correct ,after i configured my pdc with external ntp to set it as authoritative for the domain ,i  should to the following commands on the dc that is not the pdc:
- Stop the w32time service
- Running "w32tm /unregister && w32tm /register/"at the command prompt (without quotes)
- Starting the w32time service
and then they will have synchronize time between then ,and any workstation that will authenticate to the domain using that 2 dc will be clock synchronized without any additional configuration?

When you say pdc it sounds like you are referring to Primary Domain Controller. I am refering to PDC as PDC Emulator which is one of the 5 FSMO Roles your Active Directory Domain Controllers are holding. This will be either the first domain controller you setup in the domain or if you have setup as per best practice these roles will be shared between the 2 DC in your site. The DC that holds the PDC Emulator role acts as the authoritive time server and it should be getting its time from an external source

when we say pdc we referring to the same thing :pdc amulator
i asked about the second domain controller