Time is one of the great mysteries of science, and that is not because it is difficult to examine, like some distant star. Its elusiveness is due to its ubiquity in our lives. One way or another, time enters into almost every equation in science. Change cannot occur without time, therefore all processes depend on time. This makes it difficult to see the essence of time.
However, if you step back, time is merely a different form of distance. We often consider the universe to be made up of particles, such as atoms, neutrons, etc. However, we can’t observe particles or objects. The only thing we can really observe are the interactions between the particles. These are events, or phenomena, and form the phenomenal world, a different term for the observable physical world. It is more useful, in exploring time, to consider phenomena as opposed to particles. In this view, time is merely the distance between any two phenomena, whether the phenomena involved are sunrises or the absorption and ejection of quanta by electron shells.
Looking at time in this light, it becomes clear that time is relative in scope. There is no single “time-line” to which all phenomena adhere, but rather all phenomena relate to each other based on their “distance” in time. The illusion of a one-way “current” of time comes from causality, the process by which phenomena are related directly or indirectly to each other. We create a holistic view of time from this stream of causality, rather than time itself. There is no true “backward” or “forward” in time, only or perception of what causes what. Technically, causality runs both ways, as physics states that any process can be reversed, but we perceive an unstoppable progress in one direction.
By this reasoning, both time and space are qualities of phenomena in relation to other phenomena. Both time and space are completely meaningless without the phenomena which are separated by them. Likewise, without time and space, individual phenomena lose their identity, as they have no interaction with other phenomena. We only know the mass of an object through its gravitational pull exerted on other objects, which is impossible to define without time. Mass even alters at different relative speeds, with the relativistic mass dilation.
We only know our universe by the interactions of things with other things. Specifically, we only know the universe by the interactions of things with our body. If a phenomenon is not connected causally with our physical body in some way, however distant, it is essentially not in the same universe as we are, because there is no way we can be aware of it.