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Time has units. nanosecond (one billionth of a second), microsecond (one millionth of a second), millisecond (one thousandth of a second), second, minute, and hour.
We have atomic clock, digital clock, analog clock, cesium clock etc.
So what is the definition of time actually according to Philosophy?
With what philosophical tools can a Philosopher use to derive the definition of Time?
Previous answers don't provide a philosophical approach of time, so here's one.
Perhaps the most important perspective of time comes from the philosopher Immanuel Kant, as expressed in his Critique of Pure Reason. The definition actually requires a bit of context.
At the time Kant wrote this magnum opus of philosophy, philosophers were debating if knowledge comes from reason (rationalists sustained that one can know everything just by learning and thinking, practically independently from the senses), or it comes from experience (empiricists sustained that all knowledge comes from experience, which is essentially provided by the senses).
Immanuel Kant proposed a key question to solve the issue: How is synthetic a priori knowledge produced?
a) Synthetic means that is pure knowledge, not coming from a definition (for example, the word bachelor already suggests that the individual is not married, so, saying that bachelors are not married is not pure knowledge, it is analytic knowledge);
b) a priori means that it does not come from experience (e.g. we have never experienced a triangle or a number, but, mysteriously, such concepts exist in our mind; Kant calls this type of knowledge a priori). Notice that a priori does not mean previous in time. It is moreover the expression of a dependency: in order to add apples, we need the a priori concept of number; that is, addition is dependent on the a priori knowledge of numbers.
Time is precisely a case of synthetic a priori knowledge: it is synthetic (a pure concept, not derived from other), and it is a priori (we don't need the senses to know time, it is something that we just know, independently of experience).
So, Kant proposed an answer to the mysterious raising of such types of knowledge in our mind: we create them subjectively in order to make understanding possible. For your question, this means that time is a subjective production, that allows understanding possible; time is not a feature of nature; it is something that exists only in our mind. Later philosophers, in coherence with Kant's approach, tend to suggest that time is just a sequential order by means of which we organize knowledge in our mind. For example: we know the entrance, we know the hall, and we know the desks of our office, but in addition, they are ordered, so we have an experience of them in time. From a blunt perspective, time is just memory and space is just movement.
For Kant, time and space are subjective productions. Kant starts his book with this, so it is perhaps worth a read.
Now, you might say "ok, but time and space are physical concepts, Einstein's relativity would not be possible if they would only be subjective constructs". If so, you are partially right: metaphysics does not depend on science: it is the opposite.
Let me explain. Science does not search final truths: it just search for empirical truth. The branch of knowledge that deals with final truths is philosophy. So, science is absolutely dependent on our senses.
This is easier to understand with an example: temperature is not an attribute of nature (in fact, not even things exist in nature, everything would be something like energy); temperature is just a feeling, a property of what we call an object, some portion of nature. So, in order to find the relationships between temperature, pressure and volume, three laws of thermodynamics were developed. At this point, scientists noticed the problem with temperature: what is exactly temperature, excluding the concept of feeling? So, they added a law which provided the concept of temperature, that needs to be previous to those already existing for them to be true, so it was called the zeroth law of thermodynamics, which precisely defines what temperature is. With that, thermodynamics has all the elements of a science: a knowledge that describe some behavior of our experience.
So, time in physics is a concept that is dependent on our experience (anyway, there's no other way of knowing it). Physical time is a mathematical formalization of our knowledge, like temperature in thermodynamics. But we can't say that time and space are final truths. We just can't know nature in other way than by means of our senses and reason.
Kant's concepts of time and space are probably those which are still influencing the most on the making of science. For example, Google for "Kant quantum mechanics" and you'll see the importance of his approach of time and space in contemporary science.
There is really no simple or all encompassing answer to your question.
I can tell you that as a trained Historian-(and as someone who has always had an interest in the Philosophy of History. The issue of time is absolutely central to our intellectual identity and purpose. To put it more bluntly, if there is no Time, then there is no History.
However, Historians, Historiographers and Philosophers of History are not the only persons who are preoccupied and fascinated with the process of time. The topic of time has always intrigued Physicists, Astronomers, Cosmologists and Philosophers of Science dating back millennia.
For example, the Ancient Greek Pre Socratic Philosopher Zeno of Elea, was interested in this very topic, though ultimately, rejected the concept of temporal motion by aiming to convince us that Reality was static and that Time, was more illusory than actual.
One hundred years ago, Albert Einstein proposed a radical theory that Space and Time were NOT separate, distinguishable entities-(as had been previously believed by earlier Western Intelligentsias), but was actually, a singularly indistinguishable entity, due to its warped and curved configuration.
Essentially, there is no singular, universally agreeable definition of time amongst Historians, Physicists and yes, even Philosophers. The defining of time, has had (and still has), a diversity of opinions and theories which reach across the globe and span throughout the centuries. Einstein's Space Time continuum may have transformed our understanding of Cosmic time, though, the properties of Time can range from the Astronomical and the Historical, to the tiniest unit on a watch or clock.
Eva T.H. Brann has a great book on this topic called "What, then, is Time?" And in the beginning she asks: "Why don't I know what that is which I tell, save, spend, mark, waste and even kill everyday of my life with perfect aplomb?"
There are milliseconds, microseconds, nanoseconds, picoseconds, femtoseconds. My guess, if one were to ask, is that at the bottom of it all there are what are called "noseconds". However, I would imagine that it would take an astronomically incalculable amount of time to test the veracity of this assertion!
If you can consider Boltzmann both a scientist and a philosopher, then you can count the position that time is the accumulation of entropy.
As a scientist, he quantified thermodynamics. Then he questioned the value of that. So, as a philosopher, he was obsessed with Loschmidt's paradox: all physical laws seem to be symmetrical with respect to time, so time should be reversible, but we do not see it reverse. Only the Second Law of Thermodynamics contravenes that trend, and it is not prescriptive enough to be a real Law. It is more of a passive observation.
Boltzmann suggested that entropy varies continuously across four-dimensional space, and the universe experienced what we think of as time or its opposite across spans where the degrees of entropy differed. So you could consider time to be that fourth dimension itself, or you could consider time to be the difference in entropy. The time we measure has to be the latter, because the mechanisms with which we measure it are driven by processes that capture entropic decay.
That leaves all of the other parts of physics alone, and turns the Second Law into something more definite, but epistemologically slipperier.
He tried to fend off the problem of the Boltzmann Brain. If we are really just sitting at some point in four-dimensional space, feeling like we are moving because a given measurement pulls us along, what says we do not exist at just that single point. How is evolving to our current state significantly different from simply having popped into existence with a set of memories and a timeline, and then disappearing again? In fact, which of those two possibilities is statistically more likely? So, who is to say that the fourth dimension is extended, and not infinitely thin?
Boltzmann fell back on the continuity of everything else in physics. But we have since his time somewhat abandoned that notion, imagining that perhaps even space needs to be made of particles. But if you are not willing to go that far, this gives you a reasonable definition for time: time is accumulated entropy, piling up from some state of zero entropy established at the point where the density of energy in space did not allow for motion.
