Its from rocketry. T is time of launch. Anything after launch time would be 'T PLUS' whatever time has passed; so anything before launch must be 'T minus' time.
Its use is actually older than most realize, IIRC its from a 1930'* movie.
She started having contractions Wednesday afternoon; my wife was sure she was going that night. I spent all day Thursday over there and ended up taking the grandson home with me - still having contractions but far apart. I took her to the doc'* today they said she'* not even dialated; they Bracks & Hickson'* (?) and the baby is fine and won't be out just yet. I hit every pothole there was on the way home; the bumpy car ride inducing labor theory being tested here - LOL