The expression t-minus [time measurement] became idiomatic after the public was exposed to NASA mission commentaries. The popular idiom is give a humorous seriousness to a measure of time. Examples found on the internet:
"T-minus 21 days until vacation time!!!!!!"
"Wedding is in t-minus 2 hours!!!"
"Baby due in t-minus 6 days!!!"
As you've noted, these uses do not seem to respect the exact meaning of the terms in the expression. That's OK. We say all kinds of things that are apparently illogical. Here are some borrowed from a Quroa post.
Head over heels - this is the typical standing posture, whereas the expression typically means something like knocked off one's feet
Meteoric rise - a meteor is something from outerspace that has entered the atmosphere and is falling towards the Earth. Meteors don't rise
Back-to-back - means consecutive, though it's not obvious what the "back" of either event is
For what it's worth, the popular (illogical) usage of "t-minus" does not follow the typical NASA usage. In NASA lingo, T stands for "time" and denotes the time that some significant event will take place. They also use other letters, like L (launch) or E (event). You can measure the distance, in time, from the event by using T minus to mean time before the event or T plus to mean time after the event.
Here are some examples from the transcript of the Apollo 7 mission commentary:
"This is Apollo Saturn launch control at T minus 1 hour, 44 minutes in counting."
"The countdown is continuing to go satisfactorily at T minus 1 hour, 43 minutes, 8 seconds and counting."
"The standby crew must depart from the area by the T minus 40 minute mark"
"T minus 6 minutes 20 seconds and counting,
however, we just heard a report over the circuit we are
asking for a HOLD. We are standing by for further reports.
We are now at T minus 6 minutes IS seconds and holding, T
minus 6 1S and holding. This is Launch Control 6 mihutes
1S seconds and holding, the test supervisor advises he
expects this to be a very brief HOLD."