Time is a fictitious or non physical concept. Time is “measure of change” with reference to a context. A memory, “When it was so and so” e.g. modern calendar has birth of Christ as context. While for standard time, the context is position of sun with respect to rotation of earth. Only the context has beginning and end. Consciousness is by contrast-of-context and not by time of a clock. A context is a state of arrangement of internal or external of sources of stimuli for a conscious-being. Psychologically, time is product of sequence of memory, self-experienced or taught by society.
The activities of life on earth and wakefulness are governed by rising and setting of sun. Sunlight is the source of life and of visual and thermal sensations. Therefore, context of sun is seen as creator of changes in circumstances on earth. Naturally time was chosen to define with reference to changes in position of the Sun at any geographical location on the Earth.
Since rotation of the Earth is continuous and uniform in comparison to our lives, time is misconceived or wrongly associated as continuously flowing independent absolute variable. While process of life and that of consciousness is relativistic and dependent upon every conscious-being.
The common notion of time is matter of convenience of reference or context because without reference nothing can be thought or compared. Thus time is measure of change of context to individual.
The dimensional unit of conventional time is change in energy per unit of distance/cycle (displacement).