system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we say "T-minus" when counting down to an event or time?

I'm guessing T=the set time. Is there more to the story?
gpt
The military uses the nomenclature of Y year, D day, H hour, M minute, S second, and T time.  "D-Day" was really just one of many D "days" of WWII; the day when something important was scheduled to happen.  

T time just means "the time when something is supposed to happen".  T minus means "before that time", T plus means "after that time".

This nomenclature seeped out of the military and is used in a lot of engineering applications for its clarity and convenience.