Some wag said "time is what a clock measures," as if this were an answer.
The more I read about time the more confused I become. Rudy Rucker (the mathematician and SF writer) once asked Kurt Godel: "What causes the illusion of the passage of time?" Godel answered obliquely but did not complain the question was nonsense.
Recently I read Julian Barbour's The End of Time (twice), becoming more confused the second time.
I think there is a deep mystery here; something we are simply not constructed to understand (being creatures embedded in time).
Does a fish know it's wet?
--Boris
"Time is the because with which some dolls are stuffed." - e.e. cummings
Minkowski is also responsible for the common conception that we cannot exceed the speed of light. Einstein didn't seem to care much about that aspect of the theory.
A fish knows when it isn't Wet. What is the sum of all knowlege as opposed to what we can see at the present?
To me, it is the description we apply to the sequencing of events. Events and the forces acting on them can be as simple or as strange as we like them, or better said, as they are. I know it is appealing to bestow upon time some more exotic charateristics but the events and forces can have those, while time is just our necessary symbol applied to observing the progression of things.