Just as the time-sense changes subjectively and with age, both for physiological as well as for psychological reasons in a similar way the nature of time varies according to the objects considered by our mind. The time that we observe in nature has no separate existence. It is only a mode of being of concrete objects. The differences both in the meaning and in the individual perception of time, as well as its inseparability from the specific nature of any existing object characterise it as an essentially fuzzy, that is, escaping any precise definition, category. We ourselves have created the concept of mathematical time as a mental construct, which is indispensable to building up of science, conventionally comparing it to a straight line, each successive instant being represented by a point.
Time is considered as a characteristic of our motion.
If we move with a constant speed, time is expressed as a quotient of the distance we have passed to this speed. Of course, it only looks so simple. In order to explain what 'constant speed' is, we need time. Using earth rotation, people have created time-measuring unit. Again this creation is self-reflexive, as far as time has been already used when characterising the earth rotation.