If I wanted to write fiction about a time machine, I'd do this.

The machine has a chamber and two buttons.

The first button takes a snapshot of the universe - nothing immediately happens.

The second button changes the universe to match the latest snapshot taken.

With one exception: what's inside the chamber is not affected.

A fun corollary is that if you press the two buttons without changing what's in the chamber in between, you create an infinite loop and nuke the future.