Time is a delta. A means of measurement devised by mankind for keeping track and/or planning for things, fundamentally based on daily solar cycles, and the relativity of that measurement given the location of mankind to those solar cycles. Essentially, time is a man-made dimension based solely on the existence of our Sun and the position of the Earth relative to it, and then us, relative to our location on Earth, relative to the Earth's location from the Sun. Atomic clocks and other devices designed to count resonating particles and vibrations are only a means of exactly that, counting vibrations, with a set start based on again, the Earth and it's position relative to the Sun. For many thousands of years the Earth's position relative to the sun and other stars has remained constant enough to create devices that divide centuries into years into weeks into days into hours into minutes in seconds into nanoseconds.
Theories based on applied physics state that if a person was able to travel at near the speed of light or faster (~98%), they could travel through relative time. For example, if you were to somehow travel at nearly the speed of light for 1 year covering a distance around 6 trillion miles toward Pluto and then back another 6 trillion miles to Earth, you would have leaped forward in time relative to the Earth as relative to the Sun. Is this true? If so, this simply means we physically carry time with us wherever we go, and as Einstein pointed out, time is indeed relative. Time seems more like a paradox phenomena because of this reason and traveling close to the speed of light really only affects the person traveling, right? Unfortunately it does not mean we could travel a certain speed in a certain way to go backward in time and visit the past of someone else, and for us (as the traveler) it only means our time is slowing down relative to objects or people traveling at slower speeds.
In my opinion, if a method of time travel is going to be uncovered, the concept of human time will need an overhaul as traveling to the past depends on accessing moments stored along the Universal Timeline. Going to the future or back to the past will require different solutions than time created here on Earth based upon solar cycles, vibrating crystals, and measuring atomic particles. Perhaps the way is through alternate dimensions, or wormholes, or by means of telepathy. Maybe the answer is much more simple than ever as a means of projecting our thoughts into the physical realm to see the past as our brains have captured it. In either way, discovering exactly how our time keeping relates to the Universal Timeline might help us answer one of the greatest mysteries of our universe.