The past is a collection of "nows" that did exist. The future is a collection of "nows" that will exist. But this now is the only one that exists. Recording and remembering the past ("nows" that did exist, but no longer do) helps us predict the future ("nows" which do no yet exist, but will.) Energy cannot move across distance instantly, so we have the concept of "time" to allow us to remember where the energy started and predict where it will go.

But only now exists. You can't go skipping off into the future because it's not there. Neither is the past anywhere that can be visited. Only now is.