WHAT IS TIME?
From Intuition to Classical Physics
Measurement of Time Provides Possible Answers
First the event: intuitively, we can think of two events as being simultaneous - at the same time - or consecutive - one after the other.
In PHYSICS we can discuss the answer to this question by analysing the way time is MEASURED.
For example, the day is said to be the duration between two consecutive dawns of the sun. It can be easily sensed by the eyes, as the presence or absence of light.
Light/no light conditions are related to a CYCLE and based on the rotation of Earth round its own axis.
We could call it the light/no light cycle. By COUNTING the days we can define a month or a year, etc.
This would be the starting point of measuring time in a CALENDAR. All is needed is to count how many times the light of the rising sun makes its appearance.
For more precise measurement of time other cycles have been taken into consideration. An example could be the PENDULUM - a mechanical arrangement which generates cycles which can be counted, more or less constantly. This would be the BASIS of measuring the time with a CLOCK.
To answer the question now, we can say " TIME " is measured and ( for practical reasons defined ) as the COUNTING of a number of cycles.
The unit of time established by the International System of Measurements is the "second" and is defined by a certain number of cycles of radiation ( see : https://en.wikipedia.org/wiki/Time ) .
One could say that the counting process itself reflects one of the fundamental characteristics of "TIME" in Classical Physics: ITS IRREVERSIBILITY. From this point of view, time has a positive value and cannot ever 'run backwards', so to speak.