Although this question may sound rather simple at first, it cannot be answered at PTB. As an eternal mystery the time phenomenon has preoccupied the philosophers of all times. Saint Augustine admitted that when he was not asked to define the concept of time, he knew the answer but when he really wanted to give the definition, he did not know it.
A plausible definition for time was given by John A. Wheeler: It is the phenomenon which prevents all things from happening simultaneously. And thus, we are right at the heart of physics and of the question when exactly something happens and how much time elapsed between two events. It is the nature of physicists to get on the track of inexplicable phenomena by as accurate observations as possible. And this they do also when time is concerned: They build clocks which are unbelievably accurate. Being fast or slow as one may encounter with our kitchen clock is an unknown state for these clocks. Once set correctly, they thoroughly count the seconds. A date can be assigned to events. and time intervals can thus be measured. So far so good; but even the best clock does not reveal why there is time. It only shows that time exists and elapses.