Set Windows Server to use external NTP server

Stop the time service:
net stop w32time

Add the NTP servers to the manual peer list external servers:

w32tm /config /syncfromflags:manual /manualpeerlist:0.us.pool.ntp.org,1.us.pool.ntp.org,2.us.pool.ntp.org,3.us.pool.ntp.org

Set the source as reliable:

w32tm /config /reliable:yes

Start the time service:

net start w32time

Test the configuration:
w32tm /query /configuration

and
w32tm /query /status