There is an old saying that the time once passed can not be come back. So, it is better to utilize your precious time in something useful so that you won't regret it.
If we talk about the actual meaning of time then we can say that Time is a measure in which events can be ordered from the past through the present into the future, and also the measure of durations of events and the intervals between them. This is what actually time means.
If we talk about science then smallest unit, that is it's SI unit is second.