A simple and complicated answer for this question is "time is the indefinite events that occur sequentially in irreversible order". Also there is another question that "our perception about time how much differ from its reality". Some scientists believe that time does not exist and it is an illusion. These scientists see the world as indefinite simultaneous snapshots. Passing of time in different parts of the world vary from each other. It depends on the gravity forces and velocity. If you go to space and traveling close to the speed of light after one year when you come back home, you see that your friends get old more than a decade. In theory you can travel in time but your travel is in forward, and you can't experience past events. You may know that Time is money. So what do you think about the money that exist on the Earth rather than on the other plants?