If it has to save every second, then that kind of game is just too much yet for the personal computers this day. But think of it this way: the moment you turn on your time machine, the machine will actually save that moment as a point in time to travel back to later, from the future. With upgrading stuff, your handiness Sim can build in memory space for more moments to save. That way the computer doesn't have to save every second of it, just one moment. And saving the time where he came from maybe doesn't matter though, because when the traveler goes back to a previous saved moment, he changes the future anyway, just by being there. It is sort of like traveling through your older savefiles, but then with your Sim. I believe that will be technically possible and I really hope EA will put that in Ambitions.
And in case some of you ask why we will need all that stuff in the game, I can give you multiple reasons: we all know there will be disasters occurring in the game after you've installed Ambitions. Fire, gnome terror, even earthquakes. What will you do when an earthquake destroys your home with your whole collection of rare gems and expensive nectar in it? You just wish that you would have put that stuff somewhere else, before the earthquake destroyed it.
And then, you will be more than happy with that time machine and that save point from right before the earthquake started.
This way, time traveling in The Sims will be more powerful than loading up your previous save file because you don't have to loose the process your sim has gone through.
They had a time machine in the trailer a couple of posts back.
Somehow though I doubt that you could go backwards in the actual sim time; I believe more that it's just that you choose when you're wanting to go and then the guy maybe jumps back to the jurassic and comes back immediately with a reward or similar. It would take far too much memory space to save each second of the neighbourhood for you to go back to, so I think that's a bit unrealistic.
