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. <sigh> I hit every pothole there was on the way home; the bumpy car ride inducing labor theory being tested here - LOL

T minus 5 and counting.

wow you really want the baby to come out don't you.

Yeah, I feel like a little kid the week before Christmas and theres a pile of presents under the tree, all with my name on them!

Is D-Day and .. nothing. Not even a twinge. She better not pick Friday

If the sex is unknown,I bet it'* a girl

But it'* my birthday on Friday

Call him Peter!!!
haha, jk of course

Definitely a girl. They will always keep you guessing.

You can hang in there G-Dad!

Give my best to the expectant mother.

And yes, I know very well what T-minus means, being all of 20 miles from KSC.

Saturday 5:27 in the AM and the baby will wheigh in at a whooping 7lbs 8oz'*

Is there something wrong with Petra? How'd you get that name anyway?