There’s a chapter in C. S. Lewis’ Mere Christianity called “Time and Beyond Time.” In that chapter Lewis discusses God’s timeless (or perhaps it’s better to say time-free) nature. God exists separate and apart from the universe He created. Time, as we experience it, does not affect Him.
So what is time?
In Science time, along with space, is considered a fundamental, a thing that cannot be defined in terms of something else. The only possible definition is an operational one where time is defined by process of measurement and units of measure. Periodic events are used as time standards. A day is commonly defined as the time from one sunset to the next. A second is officially defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. 
As a part of the fundamental structure of the universe, time is a dimension in which events occur in sequence. Isaac Newton believed that space and time form a container for events and that this container was as real as the events themselves. He believed that absolute time flowed at its own uniform rate. Albert Einstein showed that if space and time are measured using electromagnetic radiation (eg., light bounced between mirrors), then the constant speed of light causes space and time to be bound together as a four-dimensional spacetime.
This is the point where most scientific discussions of time become even more painful than a definition of the second that has to refer to caesium atoms. Indeed, when we passed this point in my undergraduate class on quantum physics, we began to refer to the class as “Science Fiction Physics.” However, try to stick with me for just a bit more.
According to modern physics, our view of time depends on where we are in space and how we are moving through space. Both our human perception and the rate at which clocks measure time are different for observers who are moving with respect to each other. The past and the future are defined as backward and forward “light cones,” which never change. Any given moment’s past is defined as the set of events that can send light signals to the observer. The future is the set of events to which the observer can send light signals. Everything else is the present. Within the present, two observers in different locations may see past events in a different order based on different distances to events. The light cones expand as time recedes from the observer. The further into the past we look, the more we can see, and more observers will be able to see us as time moves to the future. Every point in spacetime has its own pair of light cones.
Figure 1. An Observer’s Light Cones. In the real world the present is three-dimensional. It is shown as a plane here because the third spacial dimension is used to represent time.
Time appears to have a direction. The past lies behind. It is fixed and cannot be changed. The future lies ahead, and although the observer cannot change the future’s light cone, the future is not necessarily predetermined. The direction of time is defined by increasing entropy.  The Second Law of Thermodynamics states that entropy increases over time. Another way of saying the same thing is that systems become more chaotic as time goes by.  Every observer’s light cone from the past begins at the Big Bang. However, we can’t actually see all the way back that far. Our best instruments can detect the Cosmic Microwave Background that existed about 400,000 years after creation. We can detect the lumpiness in the CMB that gave rise to the universe being organized into galaxies and stars and people. Some try to use the Second Law of Thermodynamics to show that it is unreasonable to think that the the universe could become more organized that it was in the beginning. In doing so, they are neglecting a couple of important factors.
First, the Second Law does not say that things can never become more organized. It says that useful energy is lost when organization occurs. Sure enough, as the universe has become more organized, it has cooled (that is, moved to a lower energy state). In the beginning, the universe was so mind-bogglingly hot that infinitely-hot is a close enough approximation. The average temperature of the universe is now only about 4 K (or 4 degrees above absolute zero).
Second, the Genesis account says that the universe began in a disorganized state and has since become more organized.
Back in an earlier post, I said that I would try to deal with the apparent problem of different time scales in Genesis 1 and modern cosmology’s understanding of the natural world. I actually started in another post during the discussion of Day-Age Creationism. The Hebrew word yom does not always mean “day” in the sense of a 24 hour period. It can also mean “day” in the sense of the time between sunrise and sunset. Another meaning can be “an unspecified period of time,” as in Genesis 6:5 where yom is translated as “continually” in the King James Bible.  If we follow Day-Age reasoning, and allow God to have his days be as long as He wishes them to be, the there is no time scale issue between Genesis and modern cosmology. Indeed, as noted in the earlier posts, Science describes the same events in a complementary way. In subsequent posts we’ll also consider the complementary nature between the knowledge we gain from molecular biology, genomics, and paleontology and Genesis.
For the moment, let’s go back to the thought that God is separate from His creation, that He exists outside of time. Lewis put it this way:
Almost certainly God is not in Time. His life does not consist of moments following one another. If a million people are praying to Him at ten-thirty tonight, He need not listen to them all in that one little snippet which we call ten-thirty. Ten-thirty—and every other moment from the beginning of the world—is always the present for Him. 
This might lead one to wonder about whether or not man has free will. After all, if God has already seen me typing this post, how can I not do it? The answer lies in the fact that God has not already seen the future, but that our future and our present and our past are all the same to Him. He will not know of my typing until I do it, but He is in that moment together with every other moment in our time … I almost typed “simultaneously,” but that’s not the right word. I’m not even sure what the right word would be, but I hope you’re getting the idea. God and His perception of the universe are not bounded in time as are ours.
Some philosophies and religions teach that time isn’t real but an illusion. Physicists, on the other hand, believe that time is as real as space. The Bible also views time as part of reality. It treats it as linear with a beginning at creation and with an end. The Bible shows us that God has a purpose for man and a plan to bring that purpose to fruition with the unfolding of time. Psalm 139 describes man as “wonderfully and fearfully made.”
Figure 2A. History’s Time Line. Each successive row covers a time span ten times closer to the present than the previous row. Timing is based on the best available science as of the time of posting.
 This definition refers to the caesium atom at a temperature of absolute zero (0 K). The ground state is defined as zero magnetic field.
 Entropy can be viewed as the reciprocal of information. As entropy increases a system becomes less organized, and the energy needed to restore the organized state will be greater than that lost in the disorganization.
 Consider my son’s dorm room …
 And the LORD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
 Lewis, C. S., Mere Christianity, HarperSanFrancisco, New York, 2001, p. 167.