Discovery Channel Understanding Time

Time - what is it? St. Augustine wrote, "What, then, is time? If no one asks me, I know. But if I want to explain it to someone, I do not know." Perhaps we should ask: Did time have a beginning? A very old question, indeed. But how can we imagine a world before time began? If we count backwards beyond the clock, beyond the calendar, three and a half billion years, we arrive at our own beginning -