In a Windows domain PDC isn't necessarily the domain time server. How could I identify the authoritative time server?
I'm assuming that you're looking for the server used by the W32Time service to perform time sync on domain-member computers.
In a stock Active Directory deployment the only computer configured with a time server explicitly will be computer holding the PDC Emulator FSMO role in the forest root domain. All domain controllers in the forest root domain synchronize time with the PDC Emulator FSMO role-holder. All PDC Emulator FSMO role-holders in child domains synchronize their time with domain controllers in their parent domain (including, potentially, the PDF Emulator FSMO role-holder in the forest root domain). All domain member computers synchronize time with domain controller computers in their respective domains.
To determine if a domain member is configured for domain time sync, examine the REG_SZ value at HKLM\System\CurrentControlSet\Services\W32Time\Parameters\Type. If it is set to "Nt5DS" then the computer is synchronizing time with the Active Directory time hierarchy. If it's configured with the value "NTP" then the comptuer is synchronizing time with the NTP server specified in the NtpServer REG_SZ value in the same registry key.
The low-level details of the time synchronization protocol are available in this article: How Windows Time Service Works
Beware that not every domain controller (the KDC's, as James directs you in finding via DNS in his post) may be running a time service. In a stock AD deployment every domain controller will be, but some deployments may use virtualized domain controllers that have the W32Time service disabled (to facilitate hypervisor-based time synchronization) and, as such, you would probably do well to implement functionality as described by the "How Windows Time Service Works" article if you're developing a piece of software that needs to synchronize time in the same manner that a domain member computer would.
The domain authoritative is usually the PDC emulator, in turn, other DC's will sync from it.
To determine who is currently holding the PDC emulator role in your domain, use:
For other ways to determine FSMO role holders, see the article Determining FSMO Role Holders.
Read more on the subject it the TechNet article How the Windows Time Service Works.
Some helpful commands
Re-sync (requires admin rights):
Re-sync to specific computer (requires admin rights):
Show server currently in use (requires admin rights):
Double check if its working:
Then look at the Type:
Registry settings found here:
In a properly set up Windows domain the DC that holds the PDC emulator role (there are no "PDC"s in AD) will be the time server for the domain. No other machine on the domain - including other DCs - should have a time server set. At all. Time syncing will then be managed based on the domain hierarchy, and you will have a "set once and forget" environment - at least so far as time is concerned, and until you come to move the PDC emulator role to another server.
If you need to perform any regular or ongoing maintenance on your time server setup, then something is set up wrong.