1) Stop the W32Time service: C:>net stop w32time
2) Configure the external time sources, type: C:> w32tm /config /syncfromflags:manual /manualpeerlist:”0.pool.ntp.org, 1.pool.ntp.org, 2.pool.ntp.org”
3) Make your PDC a reliable time source for the clients. Type: C:>w32tm /config /reliable:yes
4) Start the w32time service: C:>net start w32time

type w32tm /resync  (this will force a sync to the new servers)

finally, check if the server is using the new ntp server.

w32tm /query /source
Display time source