There are many concepts about time. Linear time is one. Relativistic time, in which each object experiences linear time, but not all at the same rate, as time varies when one is traveling at speeds close to the speed of light, is another.
But in philosophy and religion, there are many other notions of time, and many of those are not linear at all. In anthropology, Eliade's Myth of Eternal Return is one model of non-linear time.