How does one define the "present" tense? Other than through perception I don't think it can be defined. There is no measurement of time small enough to account for its existence. If the present lacks any tangible definition, than past and future do as well, as they cease to have meaning without the midpoint that they are supposedly anchored to. So in this sense all three are occurring at once. This is why I feel perception may be the key to time travel. I visualize time as an analogue wave that we subconsciously agree to be stuck to when we perceive its motion. Theoretically I feel it should be possible to "get off of the ride?" By this I mean perceive existence from a motionless vantage point, ie without the illusion of past present and future. The problem with perception however is that it's relative to the individual doing the perceiving. So the challenge in time travel would be to get someone to perceive themselves onto a different point on the wave, while doing it in such a way that it could be appreciated by others.