Duration is the continuous and sequential occurrence of changes from beginning to end. We perceive changes and we know that they have a beginning and an end. Time is the measure of duration.
We also realize that there are periodic phenomena. These phenomena allow us to predict with certainty when an hour, a day and a year will end. Even if we know the duration of a phenomenon we can predict its end.
The principle of certainty does not apply to the life of a human being. We know we are going to die, but we don't know when.
Time has three functions:
1- Determine the duration of things.
2- Determine the separation between two sequential events.
3- Allow to make predictions about when an event starts or ends.
In Quantum Mechanics the Certainty Principle contradicts Heisenberg's Uncertainty Principle.
