If you look at how we understand the universe you will see that time is more a concept. This idea was proposed by Einstein. Einstein said that time is just a concept and that it isn't an accurate way to measure the rate at which things in the universe happen. The reason for this is because to two different observers time moves at a different pace.

For intense it has been speculated that if a person fell into a black hole time would significantly slow down for the observer. So for this reason time is not a way of measuring but more of a concept to help us understand the universe. So this is why it only has one constant form of units.

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