I guess the simplest way is using hubble's law where velosity of receeding galaxy = hubble's constant X distance: v=hd
h is a function of (distance/time)/distance. Therefore, a series of calculations can be used to express hubble's constant as time. To reverse the acceleration, then, we need the inverse of that. I.e. 1/hubble time.
It's not particularly complicated in principle, and it's well explained here
http://manini-hlphysics.blogspot.com...4-e65-e66.html