The same as what? "At that time" or "Up to that time"?
In AusE, "By Monday" means "When Monday comes", not "Up until Monday".
"You must know the answer by Monday" means "When Monday comes, you must know the answer" not "Up until Monday, you must know the answer" (and then you can forget about it). OR "You have until Monday (up to the time [day of Monday]) to get the answer.
"By Monday, she was past caring" implies that before Monday (OR up to the time of Monday) she was building up to a stage of being beyond caring, and when Monday came, she had reached that stage. It doesn't mean she was past caring on Sunday (obviously, because Sunday precedes Monday), although that is not ruled out by the phrase (I would infer that it does rule it out in that example).