Time is not as real as we think it is. We have watched too much Dr Who. We think we can travel in time. We are confused as to whether or not the future already exists.
The truth is that time does not exist in Nature. She only knows about now.
Everything in the universe is in motion and changing. Because we can remember what it was like in the past, we have a sense of time. Nature has no memory and cannot think, so she has no concept of time.
We measure time by noticing the position of moving objects and by counting. So we used to look at the position of the hands of a clock and count off days, weeks, months and years on a calendar. While our ancestors used to look at the position of the sun, moon and stars in sky; we now have digital watches.
Nature is full of natural clocks. Our ancestors thought the sun moon and stars moved across the sky, but now we know that it is the earth which is spinning as it orbits the sun. Even the light from the sun is one of nature's clocks ticking of a second for ever 186,000 miles it has travelled.
Clocks are not very good at measuring time! All sorts of things make them 'run fast' or 'run slow'. Even the earth looses time as the action of the tides adsorbs energy and slows it spin. A billion years ago, a day was only 18 hours long.
The most accurate clocks are called atomic clocks. Two similar notes played on musical instruments which are out of tune produce a beat frequency. An atomic clock does much the same thing with light from cesium atoms and counts the beats using a computer which divides the number of beats by about 9,192,631,770 to get the time in seconds. But the exact number of beats to the second depends on the height of the clock above sea level.
If we take an atomic clock upstairs, it will run fast; if we take it downstairs, it will run slow. If we take an atomic clock for a drive, it will loose time because speed makes clocks run slow.
Einstein was wrong about time. He thought time is real and he got confused about the differnece between clocks and time. He thought that when clocks run slow, it is because time has slowed, or when clocks run fast, it is because time has speeded up.
Time is only a concept in our minds, it is not a physical reality, so we can compare all the natural and man made clocks running at different speeds and define a precise mathematical concept against which we can measure the variations in clock speeds and understand the physical effects which cause them.