Without any reason to prioritize one perspective of time over another, this means time isn't a constant universal unit at all. It is a relative measurement that varies as objects move faster or slower, or as they're subjected to more or less gravity.

Is time a constant or a variable?

To answer the question, NO. Time depends, for example, on spatial properties. The speed of light in vacuum is an invariable, but not a constant either. Time is as much independent variable as independent is the indication of a clock it refers to.

Is time a constant or relative?

In the Special Theory of Relativity, Einstein determined that time is relative—in other words, the rate at which time passes depends on your frame of reference.