In my opinion, time = change.
Without change, there is no time.
Changes happen relative to other changes, which in turn, are changing relative to yet more changes. This creates the chain of causation for linear events.
Linear events involve a beginning and an end.
Now why does change exist? Now that's the time-stopping question isn't it?
Personally, I've come to believe everything is a manifestation of mind. Including time. What is being manifested you ask? Why...everything of course.
Nothingness has never existed, and never will. You see, alot of people (many scientists and theologists alike) believe there was nothing.....nothing......and then POOF.....something. I'm contending, that this was not the case. Everything always existed and always will. Remember how I said time = change? Well, it is also my opinion that existence = awareness/mind.
Because nothingness never existed, everythingness always existed. All possible thoughtforms are and always will be manifest. The universe is the manifestation of a thoughtform. This world is the manifestation of the thoughtform of separation. All natural thoughtforms within that domain will reflect that in one way or another. Energy is not energy. It is thought. It is the thoughtform of action. Time is the thoughtform of impermanence. Energy....Matter....Time....."Indivi-duality"...all expressions of separation and...yep....duality.
Now I can't prove any of this of course. All I can say is that it is currently what agrees most with my intuition.