Why not let the user at least give an estimate for the current time & date. Should be close enough for certificate validation.

Yes, that gets annoying fast for said user, but a good incentive to eg fix the bios battery.

Imagine there is no user, and there are thousands of machines

Just have those machines randomly ask one another for the time. Because of the Wisdom of Crowds the time will eventually be right.

/ sarcasm

Fair point. Another reason I don't want to manage thousands of Windows machines. Or even one for that matter.

That's the point, if you can't trust the connection, stop trying to come up with more complicated ways to use the untrustable connection and accept that there is no safe way to update time.

Ok, so the system should fail to boot, right?

If for some reason an invalid time causes boot failure? Sure. There are some instrumented devices I work with that will alarm and cease operation if they lose their serial connection to a satellite clock, but those are few and far between.

But since the vast majority of them do not fail in that matter if the normal, configured methods of determining time aren't working, using TLS parameters that have been deliberately randomized by standard for like what, a decade now?, can not possibly be a more straightforward failure mode than logging errors and waiting for operator input.

If it's that critically important, embed a fixed signature for a set of known time sources and query those as a last resort. Microsoft certainly has the resources to set up a few to do this the much more obvious and predictable way.

That means your server could not serve HTTPS traffic at all, because you need valid timestamp for that.