To folks living near the beach TMU means "The Moon is Up" referring to the tides, as thy are affected by the moon.
But then again, when I was in flight school, our flight (platoon) "mooned" all the other flights "flight platoons," so I don't know if this has any bearing on the true meaning on the little acronym or not.
Golly gee whizz... a feller can learn a lot on Volnation threads if he just tries.