If you have a NAS on the network, you might look into whether it supports acting as a local NTP server. I run a couple of QNAP NAS’s (on different networks) that I have also act as LAN NTP servers. As far as I know, all QNAP NAS’s can act as NTP servers, while getting their own time from an Internet NTP server. Where this comes in handy in my case, is to provide time synchronization to a bunch of IP cameras on the network that are blocked (by the router’s firewall rules) from being allowed to access anything not on the internal LAN. (i.e. Out of paranoia, we don’t want the cameras to be able to access the Internet even if they were to become compromised, so the router blocks all traffic between the cameras & the WANs.) Yet we can keep all of the cameras time synchronized well enough for our purposes.