After you explain what powered the big bang
The interaction between anti matter and matter with there being more matter particles than anti matter particles, these two particles interacting caused an explosion. After the explosion pushed the matter outwards from the singularity there was a period of rapid expansion within this first millionth seconds after the big bang. If I were to unify two theories if spacetime is a superfluid then it could be believed that would be the reason for the forward motion of time since the big bang.
The below will should help to clarify, bringing both the big bang theory and illustrating the forward movement of time.
Our whole universe was in a hot, dense state
Then nearly fourteen billion years ago expansion started, wait
The earth began to cool, the autotrophs began to drool
Neanderthals developed tools
We built a wall (we built the pyramids)
Math, science, history, unraveling the mysteries
That all started with the big bang! Hey!