The built in clock of a Cisco router is not particularly accurate. That is one reason why it is not usually recommended to set a router as NTP master. I am not clear why your router can not learn NTP from an external NTP server. If there is some policy reason that prevents Internet access for NTP then you may be able to install something that would provide accurate time locally. This link might be a place to start:
Router takes the Authorative clock from its self clock.
I'm with Rick. This is the reason why I don't use "NTP master" command. The components of a router is like a stand-alone computer. The clock mechanism is software driven and therefore prone to inaccurate readings in the short-to-medium term.
Look for a GPS server which can be used as an authoritative NTP server if your network is not allowed to go out to the internet. There are a significant number of vendors that can do this. You can get one that's the size of your palm to one that you can rack mount (usually 1 RU).
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...