Improvement to Mark All As Read

It seems like Mark All As Read (MAAR) does what it says, but starting at the very moment the button is pressed. What would be more useful is if clicking view all unread would also save a timestamp, then MAAR would only perform its namesake on posts up till and not to exceed that saved timestamp.

Honestly I don't know if the internal implementation of this system even uses timestamps, but the gist is that there should be a better way to look at everything you haven't read, and then mark it all as read, without marking things as read that you didn't actually read!

I should note that I don't know for certain that this is actually an issue, it just seems to be what's going on, someone correct me if I'm wrong here!