Question

What is time?

Answer 1

$$ D = R \times T $$
In this equation Distance, and Time both are variable that can change. Time changes with the velocity. The faster something travels the slower time changes. At the speed of light time stops and no longer exists. For light or anything without mass that achieves the speed of light time does not exist.

Without space it is difficult to imagine how time could exist. The fourth dimension is part of the the other three dimensions. Before the beginning of the present universe ( The Big Bang?) neither time nor space existed.

Time is a variable it can change with both mass and velocity. Time is a part of space and is considered to be the fourth dimension of the universe.