How to synchronize time between all DC's in 1 Domain

On the DC with the PDC role, you should use the command:

NET TIME /SETSNTP:...

You should not change other machines as they need to sync with the PDC and this will happen automatically.

See https://www.experts-exchange.com/questions/22978705/Trust-DNS-problem-between-2-domains.html for a solution for UK time servers.

Where are you based?

I would set one of them to synchronise with an external Stratum 1 NTP server and the rest will synchronise from that machine.

See these
http://www.ntp.org/
http://support.microsoft.com/kb/816042

http://technet2.microsoft.com/WindowsServer/en/library/b43a025f-cce2-4c82-b3ea-3b95d482db3a1033.mspx?mfr=true

http://blogs.msdn.com/w32time/default.aspx

You should use a Stratum 2 NTP server, because you do not have atomic or satellite clock hardware.  You may need to set the flags in the registry, too.  See the question I referred to.

The answer selected is not entirely correct.  While the procedures described will work, the correct way is to set an SNTP server using the command SteveH_UK suggested on the DC with the PC Emulator role.  All domain systems - if properly configured - will synchronize their time with the PDC emulator ensuring all systems are correctly set.

Among other things, having a time on a DC that is significantly different can result in the failure of Exchange server, among other things (I know this from first hand experience - I recently moved and had one DC down for 10 days prior to the conversion to standard time (from daylight time) and when I brought the system back online a few days ago, it caused Exchange to stop functioning because it didn't automatically adjust the time as expected.

The question did not read that he had a requirement to syncronize the PDC to the outside world therefor that was not included. The net time was used because it forces a refresh from the PDC.

I agree with wingatesl that it was not a request in the question.  The problem in using the computer time is that onboard clocks often have problems, and this can set you up for future problems.  That doesn't mean you can't do it, just that it isn't a recommended solution.