In a letter to the family of a recently deceased friend, Michele Besso, Einstein wrote, "... for us physicists believe the separation between past, present, and future is only an illusion, although a convincing one." Since time is relative to the observer, it is impossible to divide it up into past, present and future in a way that is universally meaningful. In some sense, past, present and future are all there at once.
"This notion is sometimes called block time, but I like to call it the timescape because it's a bit like a landscape," says Davies. "If you look at a map, the whole of the landscape is there before you, all at once. If you add time as the fourth dimension on this map, then all of time is there at once too. The fact that nothing in physics singles out a particular 'now' is a mystery."
Incidentally, there is nothing in Einstein's theory that prohibits time travel, be it into the future or into the past.
The more you look into the past, the more what happened back then is impregnated by mistery, and lack of knowledge. The more time back, the more gaps in knowledge. Besides the few glimpses of knowledge that supernatural revelation and scientific, archeological, historical, and written texts have provided to us. The same is even more true for the future. What is, can give us a slight idea what is in front of us. And a few prophecies and divine revelation as well. But the more we look into the future, the more what might happen, vanishes in front of our eyes. We can only speculate. We know most about the here and now. The here and now gives us most evidence about the past and the future.
Jim Johnson "A metric we use to explain observed changes in the observed and measured changes in matter and energy."
Time not only changes with speed through the medium we call "space" but also with proximity to a gravity well. Consider the minor adjustments that have to be made between the atomic clicked located in Grenwich and Colorado due to the fact that one is close to sea level and the other is high in the mountains, and therefore farther from the center of the earth's gravity well. Atomic clocks measure the rate of atomic decay also debunking the "fact" that the rate of decay is constant. The rate of decay depends on the conditions (speed/location) of the atomic particle being observed and measured.