"The only reason for time is so that everything doesn't happen at once." Einstein
Time is an increment of measure devised by human beings to describe a rate of change (a ratio of motion described by one object in relationship to another). Time is not absolute.
Time by itself remains undefined. Time is a measure of motion. For motion to occur there must be "space," because if there is no "space" there is no time taken to occupy it. Similarly, if there is no "time," space remains undefined.
Einstein's theory bases physical action upon this revelation, which combines the two terms time and space in order to be logically consistent. This is a form of vector (consisting of both a magnitude and direction) mathematics verses scalar (merely having a direction or a magnitude) mathematics.
In actuality, tensor mathematics is necessary for calculating events occurring in space-time however, it remains irrelevant for purposes of elaboration in his very general context to introduce the third and fourth dimensions. Furthermore, it complicates the matter beyond that which remains easily understood.
Another item integral to Einstein's observations is "The Principal of Relativity," a term coined by Galileo in his publication "Dialogue on the Two Chief World Systems," which portrays the experiences a person encounters when inside the hold of a ship without windows in uniform motion devoid of either acceleration or deceleration.
The equivalent of remaining entirely at rest, in fact, uniform motion without change remains the equivalent of remaining at rest without some means of determining the motion (like a port hole to observe the shore or other objects in relationship to the vessel in which the observer is occupying).
In uniform motion, a person pouring water from a jug observes liquid falling into a container without missing. Similarly, a fly buzzing around the hold of the ship does not float backwards when in motion. Conversely, if the ship accelerates, the observer may see water poured from a jug missing the container below instead striking the floor behind the receptacle due to the motion of the ship. The fly may float towards the end of the hold opposite the direction in which the vessel is accelerating.
This is a significant principle in which to found Einstein's hypothesis, one so powerful he was able to state when asked how he would have felt had there been no experimental confirmation of his general theory of relativity, Albert Einstein is supposed to have remarked, "Then I would have felt sorry for the dear Lord - the theory is correct." Reasons for this level of certainty stem from its origin in "The Principle of Relativity" and his interpretation of results from Jules Henri Poincar and Hendrik Antoon Lorentz's efforts on the subject of time dilation and consequences stemming from motion at the velocity of light.
Essentially, the interpretation of the day proposed that at velocities near the speed of light, objects would shrink in the direction of motion. The reasons for this stem from selecting time as a constant in the equation and the velocity of light being a variable. Most notably, the velocity of light was ballistic meaning that if an observer were to throw a baseball as fast as possible in the direction of travel, the combined velocities would be additive.
This would mean the speed of the baseball and the velocity of light would be additive making the combined velocity greater than the constant we now know to be the velocity of light. Einstein dismissed this proposition stating that observers would not shrink in the direction of motion and moreover, since this could never be experimentally verified, it was meaningless to suggest such notions (if the observer shrank, so too, would the observers ruler therefore making it observationally irrelevant in terms of experiment).
Einstein proposed an altogether new suggestion that would rock the very understanding of the observable world. That is, he simply changed the variables in the equation choosing to suggest the velocity of light is constant independent of the motion of the source and that time was the variable. This intuitive bit of insight also eliminated the need for a medium in which to transport action at a distance. However, Einstein did not say this medium did not exist merely that he did not need it for purposes of explaining action at a distance.
In one fell swoop, Einstein removed an observationally inconsistent action (people shrinking when at extreme velocity) from observed physical phenomena and tied together results from the famous Michelson-Morley experiment (which experimentally failed to reveal a medium in which light traveled. I.e. an aether) with Maxwell's mu and epsilon (the permeability and permittivity of free space) which confirmed that light was electromagnetic and traveled at a constant 300,000.00 meters per second squared.
This raised significant questions as to the nature of time. Questions, which resulted in Einstein and Minkowski clarifying with the introduction of a new term space-time the four dimensional result of computations designed to provide for Cartesian coordinate manifolds describing the location of objects in space.
Without delving too deeply into this very complicated subject, digressing from the point of this paper, let it suffice to say the result of such efforts significantly clarified the true nature of time defined in human terms.
Most notably, the reason clocks tend to measure the exact same time when referenced is largely coincidental and reveals more about the precision of human mechanical ingenuity than any property of time.
This fact further confirmed in recent modern times by the United States Navy using Cesium atomic clocks, one left on the ground and another taken in an aircraft flown for a timed period. When the aircraft returned from its flight, it revealed that moving clocks run slow or time is not absolute.
Therefore, time is merely an increment of measure designed by human beings to describe a rate of change between two objects. While this discovery poses significant obstacles for the advancement of understanding in physics, at present, it remains consistent with experimental observations and a cornerstone in the foundation of modern understanding about the world around us. Furthermore, as results and understanding continue to clarify the world around us, the understanding of the properties of time is unlikely to change significantly. That is, time is not absolute.