How to make Windows Active Direcotory follow linux for Time sync

I have a Windows infrastructure hosted in a restricted environment. We have apps that need to be with exact time sync with the external time source. To sync the Windows AD with external time source, I hosted a Linux NTP server. Now i am not being successful in setting up the AD to sync with Linux NTP server, as i get an error "This machine is configured to use the domain hierarchy to determine its time source, but it is the AD PDC emulator for the domain at the root of the forest, so there is no machine above it in the domain hierarchy to use as a time source. It is recommended that you either configure a reliable time service in the root domain, or manually configure the AD PDC to synchronize with an external time source. Otherwise, this machine will function as the authoritative time source in the domain hierarchy. If an external time source is not configured or used for this computer, you may choose to disable the NtpClient.""

Could some one advice on how can we sync a Windows AD hosted in a DMZ infra, to sync with External Time source?

do we need to have a linux/windows instance which will communicate to the PDC??

--pasha