They know that the faster you get to the speed of light they know that the passing of time slows(as best as we can measure this). They have to make adjustments (based on this phenomena) with space probes velocities(especially going away or coming towards us) lest the adjustments made cause errors in the craft’s flight path.(Not just based on distance but on their current velocities since their onboard clocks will appear to slow down relative to Earth clocks) Another great experiment was when they took two identical timing devices, synchronized them and then sent one around the world on a nonstop flight(I think it was 2-3 times) at speeds of roughly 600 to 700 miles an hour. The onboard timer lost several seconds compared to the on ground timer(the time lost was proportional to time “0” experienced by an object at the speed of light or C or SOL 1). So time appears to be a dimensional reality or at least some type of force affected by acceleration and velocity. How gravity affects time I’m not sure, but we do know that speed and acceleration do.
So...when time slows down in space, the universe is not as old as earth?
OK, I am a visual learner and first had to find out about quarks. Then I wanted to see if they had photographed a molecule. Yup. 2013. Time for me stopped in 2012 when my kid was in Afghanistan. Restarting the clock is super hard. I’m stuck in that time warp. Good thing I was on earth though, can’t imagine time going any slower.