As strange as it may seem, time may well not exist. Take away the structure we as humans assign to time, and time slips into a category not unlike space. Space may not exist either, but that is another article. For now, let's assume open minds and play with the idea of time as a contruction of the brain.
Time, as we see it, is a useful measurement of duration. Duration is measured in the same manner as any other dimension, from one starting point to another. If we divide dimensional measurements, we get reference points such as feet or meters or nanosomethings. When we do this to time we call the deliniators seconds, minutes, hours, etc. All well and good, but all made up by, well, by us.
Universally, that won't do. Time serves us depending on our reference points. Standing still, time - let's say a second - is generally universal. If something moves, such as us or the sun, the we have some kind of spatial speed. Of course, this is simply the average of movement from still to full speed including acceleration. This "speed then, is our velocity.
Let's say our velocity is constant. What happens to our velocity if we run out of energy to fuel it? Yes, we slow down. So again we add more fuel to maintain that velocity. So we get to Grandma's house or alpha centauri on schedule, right? That depends.
It depends on time being a constant, which, sorry to say, in some cases, it is not. This leads us into the not quite settled area of quantum physics. In this Looking Glass collection of dimensions, (maybe as many as 11 of them), we find that the rules change. Electrons display rotation in one location (approximately) while existing in another, perhaps millions of light years away. That defies even Einstein's rules about space and time, but only if we accept special relativity. After all, Einstein generally reminded us of how time flies - or slows down in relation to other things.
So time is less constant than we thought. This gives us a moment (whatever that is) to think about time as something less real than we might surmise. Time then as a measurement in familiar circumstances, indeed serves us - after all we created it. In a quantum world, time may be nothing more than a concept worth toying with, but not taking quite as seriously as in our limited-dimension circumstances.
I will leave you with this thought: In order for the "clocks" on our several global positioning satellites to keep our location on the planet accurate enough to be useful, did you know they must be adjusted everyday? Yes, it is so. They have to be adjusted because they are further away from the center of gravity than we are and therefore run at different - um rates of time. Time is a trickster, nonetheless, useful in most - I said most - circumstances; as long as we keep our eye on it.