Every event in the past, present and future have already happened. We just happen to be traveling through the events on our own linear timescale.
If you're still struggling to comprehend this unintuitive idea, then think about this analogy that Perez-Giz uses in the video: "Imagine we're all reading a flipbook made of graph paper. Everyone agrees on the story, but we don't agree on which page numbers each event happens on, how many pages there are between events or the order of some of those events. Except that we are all reading the same book."
The phenomenon of measuring different time scales for the same event is discussed in the following mind-boggling video by Gabe Perez-Giz below: