Setting Server 2008r2 as authoritative time server

First, you need to locate the server with the PDC role. Open the command prompt and type:
netdom /query fsmo
   Look for the PDC line

Log in to your PDC server and open the command prompt.

Now Stop the W32Time service:
net stop w32time

Configure the external time sources, copy and paste:
w32tm /config /syncfromflags:manual /manualpeerlist:"0.pool.ntp.org, 1.pool.ntp.org, 2.pool.ntp.org"

If it does not work the first time you paste you may need to replace the " marks.  So just hit the up arrow from the command line and delete the marks and put them back.

Now it's time to make the PDC a reliable time source for the clients:
w32tm /config /reliable:yes

Start the w32time service:
net start w32time

Check the external time server configuration by typing:
w32tm /query /configuration