Time  is  not  a  independent  quantity , it is  depend on  space  variation...  i.e.  well known 4 dimension  coordinate frame , in which  one coordinate is  time.  

One of the most recent and very qualified approaches to the (physical) nature of time comes from Lee Smolin in his "Time Reborn: From the Crisis in Physics to the Future of the Universe" (2012). But his conceptual analysis of time is not very clear-cut. Nonetheless it is his big merit that he tries to re-implement time as a key notion of modern physics.

This said, you should distinguish four core features of time:

1. The ordering of all events in a before-after mode (not contested by nobody, not even the hardest physicalists)

2. The discrete order of temporal moments on a planck scale versus a continuous flow of time (not anymore contested by nobody, partly because it does not interfere with other highly controversial features of time).

3. The absolute ontological emphasis (ie. singular status) of the Now in contrast to all other moments in time (ignored or even contested since Einstein's relativity theories).

4. The structural coherence of the universal event "web" in such a way that there is only one Now for the whole universe (heavily contested by the General Theory of Relativity, reaffirmed by Smolin). If this is true, it also applies to all past and future moments.

If you adhere (as Smolin and me do) to the singular status of the Now, you have to explain what theNow really does. I suggest that it works as a universal core function by transforming one real possibility (chance) out of  many into a fixed, ie. real fact.

Hope this helps to begin with...

Saint Augustine's statement on this is certainly very important:

"What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.”

I believe time is directly related to frequency. If there are different dimensions, then they would be defined by frequency. Higher dimensions would experience time at a faster rate. Also as frequency, time would be defined by (the frequency) of virtual flux photons or other quantum particles. If there were no flux, time & activity in space would stand still. The rate of flux (frequency) defines the time, and is relative to the vibrational rate in which a person or entity is made of and thereby experiencing.

In order to provide a reference position for a discussion of time, a proposal for understanding time in the pursuit of science is presented.

Time as a component in science is visualized as the sequential progression of the matter distributions that comprise the Universe. The nature of the Universe is perceived by analyzing changes in the distributions of its matter. As a technique to manage and understand the fundamental aspects of the process, sequences of observations are divided into subsets. The discovery and creation of uniform, repetitive phenomena from the rotation of the earth to oscillations of quartz crystals provide the metrics for clocks. This allows subsets to be assigned intervals as a function of reference oscillation patterns. Integrated into this process man has formulated the concept of time.

1. Time is a concept created in the mind of man in a quest to understand the Universe. The “variety” of time under consideration here requires both matter and motion.

2. The disposition of time is continuous; i.e. not quantized, as long as matter remains in motion. The parametric devices utilized to measure time merely provide metric reference intervals superimposed on sequential observational patterns.

3. In the universal visualization, the passage of time is represented by the sequence of matter distributions manifested by the Universe. Only the present instantaneous configuration exists and it is never the same for two instants. The past configurations are remembered as the past and anticipated configurations are designated the future. Time travel in this scenario is impossible since only the present exists. However, conceptually, science is equally dependent upon present, past and future.

4. Space with a mathematical axis system may be utilized to provide individual “instantaneous” distributions of matter components. Our laws and theories are formulated from recorded observations. Their validations are tested when future becomes present. Time is the sequential progression among individual patterns. Clocks provide the units for cataloging the intervals of patterns. Recording the history of events requires the simultaneous utilization of mathematical space and time with their origins (present time and zero relative distance) superimposed.

5. On a local basis when we ask: “What time is it?” we are asking, by convention, what is the configuration of the reference clock?

6. Time possesses a mystical quality and becomes a metaphysical entity when we equate it with duration exclusive of the physical world. Newton was in this camp.

An expanded version of Time in the Pursuit of Science is presented @ http://www.researchgate.net under William Blackmon or go to http://www.pivotalconceptsinscience.com and click on PC Blog 4-Time.

I liked the answer of Wolfgang, summarizing the many problematic points.

[turns out my answer was deleted. I'll go back to edit this and propose possible solutions to all these dilemma.]

In special and general relativity, time flow in one direction from past to present to future. In periodic relativity this is considered an illusion and time is treated as a periodic phenomenon. This approach solves many of the unsolved problems in physics, metaphysics and philosophy. For complete treatment on the subject see the following article and the references therein.

http://ptep-online.com/index_files/2015/PP-40-11.PDF

The current challenge of defining time is our being immersed in it and thereby unable to gain an outside perspective, i.e. from a position where time is not.

Indeed, just as the effort of understanding whole Universe while at the same time being part of the same very Thing we wish to understand.

Wolfgang and William made good summaries of the concepts of time. At this moment, the Universal Now notion seems attractive being an explanation both relativity theories and the perception notion. If we imagine that the whole space-time of reality, the whole Universe is one big Now, then it follows we can both imagine the relativity of Einstein and that the "passing" of time is just perception of one event to another.

Ontologically, I think that can say that time is the effect of the chain of causality in the existential level. The "event" we experience must be a well-defined segment of this chain.

But e-gads, the philosophical implications are staggering; are you inferring that we might have more or less responsibility in the staging of said "events?" ;)

Dean, I have not thought of the consequences to include moral implications such as what you mention here. But I agree that together with the premise that we humans have free will, each of our actions can and does influence the whole fabric of causality. However, I don't think that human actions can contribute to the physical shortening or lengthening of time.

As for the "staging" of the "events": My idea is that each single "event" we experience is actually not singular but a segment of the causality chain, or a portion of that "fabric" mentioned above. We are also responsible at the point of recognizing the boundaries of that segment forms part that event having intellectual powers. For instance, the breaking of the window glass pane is bounded by the initial breaking of the first molecular bond and the attainment of static stability of the last broken fragment. But we can actually extend the chain of causes beyond these. Why not include the ball that strikes it? Or the falling off that vase? Even within the event, other causes may interfere with the breaking window such as interference with the prevailing wind or curtain or the window sill.

However, if by "staging" you meant that the cause-effect relation is man made, I do not agree. Simply, the postulate includes observation that our existence and each of our actions are part of the big Now, the fabric of all cause-effect. It also does not deny of our free will, it presupposes free will and our influence of the temporal future. The big Now is what we more  appropriately call the whole Existence.