If I have a time machine I hope I'm doing a bit more than buying stock in tech companies.

The present value of your efforts at time 0 (the present) would be much higher if you bought apple in time -15 and then returned to the present than if you spooled off a bunch of stock market history at time 15 and then returned to time 0 and dutifully executed your trades.

Of course, it would probably be easier to pull off the second approach (a-la back to the future 2) than the first, since opening a brokerage account in time -15 would likely cause unwanted contact with your past self.

You also run a chance of your past self discovering your plan and fooling about with it in time -14.9 - 0. I guess that lowers your expected value by a figure that depends on your probability of being caught.

At the end of the day, it's probably less of a headache to just buy an index fund.