My PDC had system event log errors:
Time Provider NtpClient: 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.
I've now run the following:
C:\>net stop w32time
Configure external time sources, type: C:\> w32tm /config /syncfromflags:manual /manualpeerlist:”0.uk.pool.ntp.org,1.uk.pool.ntp.or g,2.uk.pool.ntp.org,3.uk.pool.ntp.org”
Made PDC a reliable time source for the clients. Type: C:\>w32tm /config /reliable:yes
Start the w32time service: C:\>net start w32time
If I do C:\> w32tm /query /source on my other two Domain controllers they report back correctly with the PDC name.
Clients when using the same query report back any of the DC's.
I am unsure how the DC's and the windows 7 clients are getting this information, it seems not be via DHCP or Group Policy....