Update 2018-Oct-03: This post has been updated within new information from NDcPP v2.1.
Recently, NIAP issued Technical Decision TD0321: Protection of NTP communications. It states that network time sources are critical pieces of information that must be protected. However, having no other agreed-upon mechanism to authenticate the source of, or ensure the integrity of NTP packets, NIAP requires vendors to use NTP over one of only a handful of acceptable trusted communications channels: TLS, DTLS, HTTPS*, SSH or IPSec.
This leaves many vendors in a bind since there are (a) no public-facing NTP servers that operate over any of these permissible channels; and, more importantly, (b) there are no widely available NTP server/client implementations that can be used to build such a solution.