How do you know? Maybe you'll win the lottery next week.
I could go back to a day in the past and win the lottery then, and return to the present with many memories of how I enjoyed my winnings.
That'd be cheating, the past is already written but the future is a blank page and we all hold tiny pens.
Long as I remember The rain been coming down. Clouds of Mystery pouring Confusion on the ground. Good men through the ages, Trying to find the sun; And I wonder, Still I wonder, Who'll stop the rain.