A clock is just a machine that ticks at fixed intervals. We may say that it measures time, but it doesn’t give any information about what is the time itself. The flow of time and the way we experience the existence of time may be just an illusion.
Past, present and future, all of them may exist right now, at this moment, but in a different time frame that we can’t access in our normal life scenarios.
Watch out awesome explanation by Brian Greene in this video.
Space and time are inseparable
We cannot look into space without looking back into time. Our advanced telescopes can see billions of light years far, they are not looking only far into the distance, but they are also looking into the things as they were billions of years before.
In relativity theory the past, present and future already exist in the four-dimensional continuum of space-time. We think next weekend hasn’t occurred yet, but in relativity, every event of our life has already happened in a single 4-D object called our world-line.
In the words of Einstein:
Since there exists in this four dimensional structure [space-time] no longer any sections which represent “now” objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence.