Time is something we take for granted.
We notice its passing both in absolute terms and in duration.
That is, we know today is the day after yesterday and the day before tomorrow.
We can assign days of the week, days in a month, months in a year, and years from some arbitrary starting point.
It establishes for us an absolute context although not immutable.
Duration is measured in moments, in seconds, in minutes, or sometimes forever.
Duration is the stuff of our daily lives as we consider how long it will take to drive to the store and how long a lecture is going to be.
Our lives are measured in duration with the average human having some 2.5 billion seconds to spend.
Philosophers, historians, scientists, poets and priests have all considered time and its implications.
From the ancient Greeks, Plato, Aristotle and Socrates, to modern philosophers such as Daniel Dennett, time has remained an elusive concept.
In the last few decades, psychologists have identified the time circuits within our brains which allow us to understand day-night cycles and to measure duration.
The complex relationships of different components in the brain and the neurological pathways connecting them result in our sensation and understanding of time.
But we are still left with the question of why time is?
Perhaps the simplest explanation was offered by physicist John Wheeler who said "Time is nature's way of keeping everything from happening at once."
Time happens regardless of whether there is someone to observe it or not.
It is not a human invention but fundamental to the universe.
Hence, we can say time began with the Big Bang even if no one was there to watch it. (Of course, how could they be?)
From a physical perspective, time can be observed in any number of ways from the frequency with which atomic nuclei spin to the excitation of electrons.
It can be observed in the frequency of vibrations with the electromagnetic spectrum.
It shows up in the time it takes atoms to make or break a molecule; chemical reactions take time.
It can be measured using the radioactive decay of the elements.
This is how Clair Patterson became the first scientist to accurately age the solar system.
He was able to look at the relative amount of uranium and lead within crystals of zircon.
As lead is the decay product from uranium and the different isotopes of lead arise from the different isotopes of uranium with different half-lives through different decay paths, the age of a particular crystal can be determined with reasonable accuracy.
The result for meteors is 4.55 billion years give or take a few million years.
And the implication from this is the solar system coalesced around that time.
The difficulty with actually measuring the age of Earth itself is the surface of the planet is constantly evolving.
Plate tectonics means surface rocks are constantly being formed at ridges and destroyed in subduction zones.
That said, some exposed rock faces in Greenland and other locations date back to about 3.8 billion years ago.
We know this age with reasonable accuracy.
But while physical phenomena provide a measurement of time, they still don't explain why time is in the first place.
We do know it is not the "absolute, true, and mathematical time, which of itself, and its own nature, flows equitably without relation to anything external" as proposed by Isaac Newton.
Einstein and other physicists have shown time is not a constant but dependent upon velocity.
As an object approaches the speed of light, time slows down.
At the speed of light, it stops entirely.
Put another way, at the speed of light everything does happen at once.
Modern physics recognizes time as simply one more dimension - spacetime is a four-dimensional construct.
Some modern theories, such as string theory, postulate there are even more dimensions to each point in the universe with some of the dimensions wrapped up into themselves.
However, even with multiple dimensions, an explanation for why time occurs is still missing.
The best explanation so far is the entropy of the universe.
The universe, as it expands, is progressing towards a more disordered state.
This disorder is measured by the entropy of the system.
And all processes tend to maximize entropy overall.
As the progression towards a disordered state is irreversible, this provides an arrow to the direction of events within the universe and manifests as time.
This does mean time has a direction and is fundamental.
Of course, if time is progressing but runs at different speeds depending upon velocity then how does it relate to the overall entropy of the universe?
Time is something we take for granted every day.
But it is complicated and maybe one day we will fully understand it.
Of course, only time will tell.
