Don't focus on Millisecs(), you are only worried about time elapsed since the last time you ran the game logic.
something like the pseudo code below...
when pause key is hit:
when game is finally un-paused:
something like the pseudo code below...
when pause key is hit:
elapsedTime = millisecs() - lastTime; store the elapsed time. (Last time is whenever you last did a 'tween' or checked the time.)
when game is finally un-paused:
lastTime = millisecs() - elapsedTime; set the current time count back by the amount you stored previously.