What is time?

It's 12:50 pm, so simple right? One can look at his/her watch or computer screen or phone to tell what time it is. But, how did human measure time a thousand or even a million years ago? There was no such thing as watch, phone, or computer. The old method is using the position of the sun on the sky; a new day begins with the sun rises and the cycle continues for what seems to be infinite relative to our life time. Another method used by Chinese calendar was the moon. One revolution of the moon around the Earth was one lunar month. Then, there came the mechanical clock, then scientists used speed of light to redefine the definition of a second, and now they are using the radioactive property of Caesium-133 to define the nature of 1 second. However, for some people, one second is merely greater than the time it takes to blink, thirty minutes is the time to get to work, five minutes is the time to brush ones' teeth, and as for me seven minutes is the time it takes to ride my bike from my dorm to my class. As we all see, time is compared/seen as distance between home and work, the position of one celestial body relative to another, the time its take for one photon to go 299,792,458 meters… If one has a hard time imagining the "fourth dimension"-time, it is, in these cases, being estimated by the other three dimensions. Nevertheless, why can't we "see" time nor draw a world with four dimensions.  That's because we just simply can't. For example, if we could see the famous, extra "fourth dimension," time, as any of the three other dimensions, we would easily look back in time and even at the future as simply looking down the road. I personally think it is really stupid of anyone to try to understand the computer graphic of four, ten, or twenty dimensions, we just can't PERIOD. As I just said, it's one reason for humans to use the three dimensions to estimate time.

So, where does time come from? People are discussing that the Big Bang created time, but how come? Don't we have time before the Bang? It is hard to perceive not to have time, isn't it? But think this way. The big Bang created stuffs which in turn created the Stars, the Sun, the Earth, the Moon, photons, Cesium... They are our instruments to measure time, aren't they? It's the creation of 3D space that makes it possible to estimate time. Not fully persuaded/understood? Put it this way, I am about to prove time backward. Assume that the Big Rip is true (The Big Rip is a cosmological hypothesis first published in 2003, about the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, is progressively torn apart by the expansion of the universe at a certain time in the future). So particles (photons, quarks...) will be infinitely away from each other. In other word, no two particles will ever meet again. So, a photon (light particle) will be alone in the universe, and it won't know that it is traveling at 300 million meters per second because there is nothing around it (such as houses, planets, or stars) to tell how far it goes. Or this photon is being confused by asking "am I really moving? I don't see so." The 3D space is annihilated into 0D space-a single point, and frankly time has disappeared because there is no photon can tell the time or Cesium to decay. So the "rip" of matter brings the disappearance of time. Now, we go back and say, the creation of matter, the Big Bang, creates time.