The following steps can be used to configure an authoritative NTP Time Server to synchronize with an external NTP time source.

Edit the following Windows registry entries:

Change the time server type to NTP
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Parameters\Type
Set to NTP
NTP stands for Network Time Protocol

Set AnnounceFlags to 5
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Config\AnnounceFlags
Set to 5
Announce flag values:
0 Timeserv_Announce_No, Reliable_Timeserv_Announce_No. The domain controller does not advertise time service.
1 Timeserv_Announce_Yes. The domain controller always advertises time service.
2 Timeserv_Announce_Auto. The domain controller automatically determines whether it should advertise time service.
4 Reliable_Timeserv_Announce_Yes. The domain controller will always advertise reliable time service.
8 Reliable_Timeserv_Announce_Auto. The domain controller automatically determines whether it should advertise reliable time service.
Flag 5 (1+4) means “The domain controller always advertises time service” and “The domain controller will always advertise reliable time service”.

Enable NTPServer
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ W32Time\TimeProviders\NtpServer\Enabled
Set to 1

Specify the time sources
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ W32Time\Parameters\NtpServer
Set servers i.e. time.windows.com,0x1 ntp2a.mcc.ac.uk,0x1
You must append ,0x1 to the end of each DNS name. If you do not append ,0x1 the changes made in the next step will not take effect.
Note: I found time.windows.com to be extremely unreliable source. I normally use uk.pool.ntp.org (in the UK).

Select the poll interval
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\ TimeProviders\NtpClient\SpecialPollInterval
Set to 384 (900 in dec)
Interval in seconds when server will poll time from the source. If set to 384 (900) server will poll time every 900sec (15 min)

Configure maximum allowed positive time correction
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\ Config\MaxPosPhaseCorrection
Set to e10 (3600 in dec)
Time in seconds. Maximum positive time correction server will accept from remote time server. Recommended 1 hour (3600sec).

Configure maximum allowed negative time correction
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\ Config\MaxNegPhaseCorrection
Set to e10 (3600 in dec)
Time in seconds. Maximum negative time correction server will accept from a remote time server. Recommended 1 hour (3600sec)

Restart Windows Time service or at the command prompt execute:
net stop w32time
net start w32time

Windows Server 2003
Windows Server 2008
Windows Server 2008R2