There is only now
I am not going to trash my clock, but have you ever experienced a time other than now? This universe is not like a movie. There is no huge pile of past laying around somewhere, nor are there infinite piles of future. Now is not a frozen time frame in between the future and the past. Don't get fooled by your brain.
Past & Future are tools of the brains
Every human has a personal virtual reality, a very useful tool of the brains. In this virtual reality, we create a vitualisation of a: past, now, and future. The now is about about observing what is happening now. The past is made of memories. The future is made of predictions, based on patterns from memories. What has happened can not be remembered exactly, but we try to remember the important parts as well as we can. The whole memorizing this works mainly automatic. The things that made a lot of impression and the memories you keep looking at, become a strong patterns in your memory. The ideas of what might happen in the future, are predictions based on patterns found in memories.
It's healthy to just observe and accept the now most of the time (yes, I'll keep taking about time, otherwise the text becomes hard to read). Memories are useful for recognizing the now, remembering what has happened, and for guessing what might happen. We have the ability to visualize a goal and work towards it, though success surely isn't guaranteed. Many people find it hard to focus on the now, to momentarily let go of projecting, the illusion of the personal virtual world. Are you scared for "the sound of now"? I am sometimes. Our brain can be quite a trap. Some Buddhist might be scared of the opposite, thinking that using their virtual world is a bad thing. Sitting still though will not get you closer to the now; there is only now. But it can help learning to focus on the now, minimize projecting for a while. I guess the most desirable is the ability to freely switch between the focus on now and whenever.
So, what is time?
Forget the illusion of a past and future. Time is about the speed of energy, or if you prefer; how fast a clock is ticking. It's about how fast shapes can change, relative to for example light. A measure of possible change. Everything that contains energy, acts like energy (cause and effect, acceleration, balance, resonance, things like that). I'd rather not call time a dimension, because it can't be traveled. Only in the virtual reality of the brain should we call it a dimension.
The speed of time
The speed of time is depending on the speed of energy, and the speed of energy is depending on the condition of space. I don't exactly know what all is proven, but I think the effect of gravity on the speed of time is proven in practice (not just theory anymore). Speed and the condition of space are relative, so anyhow; time is not a constant.
If in one region all energy would freeze, absolutely nothing would happen there, time would stand still... That is; relative to another region where the energy didn't freeze. But it's impossible to tell your speed of time if you can't compare. If the universe is expanding, maybe time is also, and an hour today is much longer than 1000 years ago. There is no way to tell, is there?
"The distance light travels through standard vacuum space in one second is exactly 299,792,458 meters. The amount of change happening somewhere can be compared to that". Troublesome it becomes if true that the speed of energy and the condition of space aren't constant everywhere in this universe. Then time becomes relative, relative to the condition of space.
The beginning of time?
Possibly did our time start at some point, but that doesn't mean there was nothing happening before that. Remember there really is no past and future, so to think there was nothing happening before the beginning of our time region, is kind of silly, tricked by the mind again. No, I don't know what happened in the beginning, I wasn't there.
No time travel
Two equal clocks can run at different speeds, depending on where they are. There's no time traveling going on though, just differences in the amount of change. Twins could get separated, and when they meet again, one could be much older than the other, but they both got older, none of them became younger. I don't know if the character of energy and space can become opposite, reversing the cause & effect as we know it. But no matter how fast the energy goes or not even in what cause & effect direction, it is always now.
Weird and cool is that all things that were or will be possible, are possible today, because there's only now. We don't have to wait for the future! (possible, I did not say "easy to realize")
The speed of time, mentally.
That's a different story.
The slower your brain works, the faster time flies.
If very little is happening, time passes by slowly.
If very little has happened, time passed by quickly -looking back.
If a lot is happening, time flies.
If a lot has happened, time passed by slowly -looking back.
"It's as if time stood still here" people say when at a location where shapes didn't changed much. "that men was far beyond his time" if he was busy with shapes that became common many years later.