valid NTP sources

I've found with NTP issues before and have a better understanding. But what I haven't been able to find is what LTM considers a valid NTP server. I originally thought that domain controllers would work, but it seems like it doesn't. So I'm not sure what qualifies as a valid NTP source. Most of my deployments won't have internet access on the management vlan, so going out isn't available.

Any thoughts?

Thanks