The phrase is an exclamation marking the event that the user needs to take a massive dump, often one that is minutes or even less to where shit hits the fan, quite aptly.
It's origin stems from the "T minus ____ seconds/minutes/hours etc.." announced for important events, such as New Year's ball dropping, bombs going off, or in this case a shit-storm.
When T Minus is yelled, followed by a time for more accuracy, it is imperative to get him/her to the nearest restroom or else all hell will break loose, or just their bowels perhaps.
John: "Shiiiiiit, T Minus! That Chipotle definitely ain't sittin' well."
Frank: "DRIVE FASTER MATT! We can make it to that 7-11's bathroom in time!"
Matt: "Man, they're gonna need a new toilet soon."
John: "I... can't breathe.. Frank.. if I don't make it, tell my family how I died.."
Matt: "You're gonna be OK! Stay with us!"