Assuming everything is working properly you shouldn't need to include any
time sync commands in a logon script as a PC should automatically sync its
time with the authenticating DC. DCs in turn authenticate with the PDC
Emulator in its domain. The only machine in the forest that needs to be
manually synchronised is the PDC Emulator in the forest root domain, you can
use the net time /setsntp command on this one machine to sync with an
external time server and that time should flow down to every server and PC.
If this process is not working, it's best to troubleshoot why rather than to
start manually setting the time.