Reverse Time flips the order of keys within the selected time. You can reverse time by scaling a selection past its left edge, but this also changes the position of the selection and the remaining keys around it. Use Reverse Time to reverse keys within a designated block of time.
(Select Time ) to specify a block of time.
(Reverse Time), or choose Time menu
Reverse.
The position of the selected block of time does not change, but 3ds Max reverses the order of keys within the block.