P. By (+a time) =not later than.
Q. We use until (or till) to say how long a situation continues.
Q1. Something continues until a time in the future.
Q2. Something happens by a time in the future.
R. can say ‘by the time something happens’.
R1. can say ‘by the time something happened’ (for the past).
R2. Also by then or by that time.