Time is such a fascinating topic. The same people who never seem to have enough time will waste time. Time which is a limited commodity in the lives of people is treated as limitless when used in small quantities. Getting a handle on just what time is has occupied a lot of time in the minds of some of earth's greatest thinkers. During the last century of so, the idea of treating time as a fourth dimension has gained in popularity. To understand time as a separate dimension requires some effort and consideration of what is involved as time is factored into the 3 spatial dimensions.
A dimension is a quality that defines existence.
If something were to exist without dimension, it could be no more than a point. It has no length, width, or height. When a single dimension is added, the point can be pushed along that dimension. Essentially, this will create a line. It will have length as its only dimension. The line can be straight, curved, or any other option. It cannot touch itself or a second dimension will be added. Of course, the line would not be able to grasp this because it can only perceive itself in one dimension.
A plane has two dimensions.
If a second dimension is added, the line can now be pushed across the new dimension and create a flat surface or plane. The plane can twist and spiral. To understand this, imagine unrolling a long piece of paper. You can grasp each end and twist the paper forming a corkscrew shape. The surface of the paper will still be flat and two dimensional. The plane could even loop back and touch itself without being altered.
It takes 3 dimensions to make a solid.
A plane can be pushed upward or downward and produce a solid by adding a third dimension of height. The universe is defined by these three dimensions in one way or another. Humans as 3 dimensional products experience everything in this way. If there are more spatial dimensions, it is impossible for humans to have a total understanding because they do not possess the correct sensory devices to perceive them.
Time is not a spatial dimension by human understanding.
If time defines space, humans have not yet figured it out although many like Einstein and Hawking have tried to do so. To consider time as a dimension requires that it be viewed as a special case that does not quite fit the mold of the other three dimensions. It is impossible to draw a picture of time. Time cannot be stretched out to cover a flat surface. It is not used as a drape over solids.
Time is spoken of as if it has spatial qualities.
People discuss time lines. Efforts are made to make time seem like it has length as defined by a one dimensional drawing. When a task needs to be completed, people speak of having enough time to cover it. In reality, time and the task will never meet and shake hands. Time is discussed as a solid when someone is concerned about filling time. By human understanding, time cannot be filled because there is no knowledge of what could actually fill it. None of these traits fit time as a dimension.
Very few definite attributes of time are clearly understood.
Einstein opened the door to the idea that time could be bent or warped. His theories also led to the understanding that time is relative. As velocity approaches the speed of light, time slows down. To the traveler, time would not seem to change for the traveler or anyone else travelling in the vessel. If they were on a round trip, the travelers may only experience a few months of time while others not traveling at that speed would have experienced decades and possibly centuries. No spatial dimension has any property near this unusual.
There are more practical applications of time as a dimension.
Pick out a patch of earth. For conversation's sake, put that patch in a place that takes in Stone Henge. Over time, that same piece of real estate has seen a lot of activity come and go. It all happened in the same place, but at different times. If it were not for the dimension of time, a visitor to the area could see a place where there had never been people. An ice age would be in process and a warm humid period would be concurrent with it.
Druids would be walking around in the climactic confusion while workers would be transporting and erecting huge stone blocks. Ceremonies would be taking place while the monoliths were showing the wear of generations. Archaeologists would be digging out pits trying to figure out what the druids were up to. Thanks to time, these events each had their own place to exist in the same patch without the confusion that would have happened without time.
Time allows other dimensions to be better utilized.
By applying time, meetings can be arranged so that everyone is there together. Time helps to prevent plane and train wrecks by keeping them from being in the same place at the same time. Time helps bakers know when to move the cake from the oven to the cooling rack. Rates of speed are always defined by the amount of length covered per unit of time.
From the earliest of civilizations until today, society has used time as a dimension to define its activities. Beginning with tracking the seasons and moving on to sundials and water clocks, people have recognised the role that time plays in their lives.