Time can be variable. Compared to clocks on tall buildings, clocks on the ground go a little slower. For instance, there is a difference of about 10 billionths of a second a day from the top of the Sky Tree, the tallest building in Japan. Although this difference is only momentary and of little relevance in our daily lives, for example, when it comes to space, we need to take into account and calculate the difference in the system of clocks on satellites.
Time stretches and shrinks according to Einstein’s famous theory of general relativity. Even more interestingly, the theory of relativity states that not only time but space also stretches and shrinks. Relativity has been proven and experiments have confirmed that time and space are not invariant.