Time is imaginary, invented, our own making. Yet without time, life as we know it could not exist. It is part of the math and science that has given us today's technology and so much more.
This short (6 minute) video tells us about how time is determined and how accurate it is. Our current system measures 16 decimal places; this means that in 300 million years not a single second would be lost or gained.
A second is exactly 9,192,671,770 periods of oscillation of an undisturbed cesium atom. And time, well in my definition of time, is that it's a coordinate that lets us most simply understand the evolution of the universe. But that is a circular definition.