B1. On time: punctual, not late. The opposite of on time is late.
B2. In time (for something/ to do something) = soon enough. Can say just in time (= almost too late) The opposite of in time is too late.
B3. At the end (of something) = at the time when something ends: at the end of the month; at the end of the film. Cannot say ‘in the end of ~. So cannot say ‘in the end of January’ or ‘in the end of the concert’. The opposite of at the end (of ~ ) is at the beginning (of ~)
B4. In the end = finally We use in the end when we say what the final results of a situation was. The opposite of in the end is usually at first.