We watch movies and play games, in which a variety of guardians of the galaxies move from planet to planet, from galaxy to galaxy. How do they know the exact time? They don't have to constantly tweak the wheel on their "switch"? Or somewhere is a giant Greenwich across the Universe? What is the time in our world?
Einstein said everything is relative. We know that everything is relative. But in physics, that "everything" is relative. Relative to the speed of light, which is constant relative to any reference point. So, let's examine.
If you Shine a flashlight in front of you, it can be noted that the beam of light moves with a speed of 300 000 km/s, i.e. it is the speed of light. If you're moving at 200,000 km/s faster than me, and Shine the flashlight, I see the light from your flashlight moving at the speed of 300 000 km/s. This is from my point of view. From your point of view, apparently, it will move with a speed of 100 000 km/s.
But if you want to measure the speed of light from your flashlight for you, also its speed is 300 000 km/s. Maybe change the speed at which you perceive time? For you it will seem normal, but from my point of view, your time will go slower. We agreed about how fast the light is moving in kilometers per second, but has not decided how long the second. While not agreed on the length of the meter.
It seems strange, because we have the space and time appear to be absolute things, and light is something that moves through space. This is our experience. If you think that time and space are stretchable and movable concepts, you can gently move the mind.
The funny thing is that the absolute is the light and the space and time are relative. Therefore, the time depending on your point of view, and therefore there is no single absolute time.
Thus, because of relativity, each point in the Universe perceives time at a different rate. When we observe the cosmic microwave background, we find out that we are moving at a speed of 630 km/s relative to the background. This means that we experience time slightly slower than what would be at rest relative to the cosmic background.
Slightly slower, but considering the age of the Universe, our cosmic clock is behind by 30,000 years. So you can regulate your watch.