I've never been much of a believer in EMC2 because it can mean anything anybody wants it to mean.
But Einstein said time is relative and this makes some practical sense.
He said that if you have two objects and one is stationary and the other is moving; time will be experienced faster for the object that is still and slower for the object that is moving.
It seems to make intuitive sense and it is experienced as a reality in life.
If we 'get stuck in a rut' time will fly past.
If we do things, experience things, learn things and keep our 'dreams' alive, time will slow down.
I don't know if you have ever experienced time slowing down in a sudden emergency?
I have and time appears to almost stop.
The last time it happened to me was in the New Forest when I had a high speed fall from my bike.
Time almost came to a standstill and I was able to relax and crumple into the fall whilst at the same time pushing the bike away from me so that it fell on to the soft grass verge...
Even thinking about it now, the second or so of the fall seems to be without time. Almost as if time was suspended.