Is time real or relative? The simple answer is, the answer cannot be known; therefore, it must be relative.
Heisenberg demonstrates that we cannot know anything to any degree of certainty. To know something, we must measure that thing. The way we measure a ting is to compare it to some known. That comparison requires that we determine if the thing is more than or less than or equal to the known. The closest measurement we can make is equal to one quanta of light. Therein lays the Heisenberg Uncertainty Principle.
Light is a very interesting thing. To understand it in one way, we describe it as a wave. Yet, handling light as a wave does not explain its mass properties. To understand light completely, we must also describe it as a particle. This single particle of light is named a quanta of light. We can only know what happens at the level of one quanta of light. We are in the dark between one quanta of light and the next quanta of light; thus, we are uncertain what has occurred between the successive quanta of light.
On a more coarse scale, consider a video made at standard recording speed. If you sufficiently slow a video of a person walking so that you are viewing a single frame at a time, you will see the person's foot on the ground, lifting off the ground, moving up and forward, and so forth. What has occurred between the frames? Even in the highest frame frequency available, there will be a frame where the foot is in one position and the next frame will have the foot in another position, and there will be a gap between the frames where the foot is presumed to have passed. Did it? That cannot be known by the evidence.
It might be suggested that if, in one frame, there was a string in front of the foot, in the next frame, the foot was beyond the string, and the string is broken, the foot must have passed through the space. That is merely a circumstantial assumption. There is no evidence that a pterodactyl did not appear and delicately snip the line with its dragon toothed beak and then realize it was in the wrong time' and morph into a butterfly.
That argument is not presumptuous. Time is measured as the passage of something. To argue that time is measured as the passage of time is circular logic and not allowed; a thing cannot define itself. If time is the passage of something, it must be the passage of events. The question is are those events continuous or are they a series of discretely related instants?
The answer must be that the answer is only knowable to the extent that it can be measured. The precision with which something can be measured is limited by a single quantum of light. Light, at the level of quanta is a particle; matter. Mater is most probably organized energy (String Theory 101). Something smaller than a single quantum of light exists in order for that single quantum to exist. What that something is cannot be known and is only presumed by the expression of the single quanta of light it produces, the single quanta of light is relative to that something that expresses it.
Thus, time can only be said to be Real' far above our ability to measure the passage of events. To say that time is Real' below some finite level requires the assumption of observed effects after the fact.
Therefore: Being unable to measure an event after the fact means that we can only measure effects and must presume cause. We must make a decision based on the relationship between cause and effect. This relationship clearly demonstrates that time, the passage of an event, is relative and Time is Relative.