Yes, they can block me, but that's just annoying, not insecure. Intercepting unencrypted stuff is the reason for this whole thread. And if they change my computer's date, they might get one malicious cert working - assuming they can change it enough, since NTP has mechanisms for avoiding large leaps - but they'll break every single HTTPS connection besides that one, which I'm sure the user would notice almost immediately.

Meanwhile, routers are generally known to be insecure, and who knows how many viruses my guests bring onto my Wifi network when they ask to connect.