I am trying to find out how to reverse the unit step function. The closest I could find is this sentence, which is more like a definition? "if we want to reverse the unit step function, we can flip it around the y axis as such: u(-t). With a little bit of manipulation, we can come to an important result: $$u(-t)=1-u(t)$$ I completly get how 1-U(t) would be positive 1 until t=0 and then it becomes 0... But how does t change to -t?