Philosophers and scientists have been trying to understand what time is since antiquity, and one thing that they have found is that it's incredibly difficult to say anything about time without already presupposing it in some sort of viciously circular argument. Nevertheless, curiosity drives us to ask questions that we then have to test by conceptual analysis and by empirical means, and we often find out that these come in conflict with each other in ways that elude simple solutions.
There are philosophical accounts of time (and I'll provide a short introduction soon), as well as scientific accounts, most of which are based to various extents on some of the incredible insights to come out of Einstein's theories of special and general relativity.
So fasten your seat-belts as Brian Greene takes us on a tour to understand some of the deepest of mysteries: What is time? Does the past exist? Does the future? Can we change the future? Can we travel back in time? If so, can we change the course of history? And hey, if you can go back in time and meet your former self, which one is really you? That last one is more of a philosophical question, just to show you how badass we are :)
If your head isn't spinning by now, then you might want to get yourself checked into a hospital :)
And if you want to watch the original from NOVA, here it is: