According to Einstein, we experience time at relative speeds.
By infinitesimal amounts, someone who is sitting down is actually aging more slowly than someone who is running a marathon.
It's not just a thought experiment with no real-world consequence -- GPS satellites have to account for time relativity to work properly, because time is literally moving more slowly on the satellite than it is on Earth.
That means that scientists working on the International Space Station return to Earth younger than their colleagues on the ground, because time moves more quickly on Earthy on Earth than it does in space. It's an almost immeasurably small amount, of course, but that doesn't stop them from being smug about it at your 40th birthday party, does it?
Then there's the fact that nobody really knows why time seems to move in only one direction to us, because as far as physics is concerned, the "arrow of time" is irrelevant to studying physical processes -- math doesn't need time (math got aaalll the time in the world for you, baby). In that sense, the fact that we can remember the past but not the future seems to be entirely arbitrary. Scientists have spent a lot of brainpower trying to figure out why that is, but they're only human, abstract concepts are pretty difficult, the new PlayStation looks awesome, and we haven't made much progress on this whole time thing. You don't need a scientist to understand how that formula works.