Don't Panic

In The Good Place, Janet is a non-dimensional being of unlimited power and omniscience, tempered only by a set of guidelines that dictate her operational parameters to serve specific functions. She professes at several points throughout the serious that she “literally knows everything.”

So...

That would mean that Janet knows the future.

Hear me out.

Per Newtonian mechanics, if we were to know the velocity, charge, orientation, direction, and all other aspects of each atom and subatomic particle, presuming an ability to correctly and quickly collect, process, and calculate that data, we could determine all of the infinite interactions that can and will occur in order to accurately predict the future.

The breakdown comes from quantum mechanics, where Heisenberg's Uncertainty principle says that we cannot know more than one attribute of a particle at a given time.

But Janet blows all that right out of the water because she “literally knows everything” and has the omnipotent power to consume and solve all that data.

The show further represents time as a single-line-drawn, closed loop on a two dimensional plane that looks something like the signature of a person named “Jeremy Bearmiy.” Janet indicates knowledge of specific events at exact points on this “line” on multiple occasions.

This means not only that events as we perceive them in “time” are limited to a specific scope, but that Janet has already seen them all, “past,” “present,” and “future.”

So she has already existed at all points in “time” while having the power to know everything, and she is capable of measuring anything and everything instantaneously in order to arrive at an accurate prediction of what will happen at any of those points.

Tada.