To sum up the plot, Doc Brown has created a time machine (out of a DeLorean, of course, given its all-steel construction and convenient space for a flux capacitor) and, by accident, Marty goes back to 1955.
By the end of the film, Marty returns to 1985 and, in true Frank Capra-esque fashion, he is a better person, so Doc decides to visit the future. When asked when, he says "About 30 years. It seems like a nice, round number." Which makes sense, since they had just gone back in time 30 years and has a certain consistency.
But the Internet today is abuzz that today, July 5, 2010 is the day Doc Brown visited before returning to Marty the next morning telling him they had to do something about his kids.
If you can add and then subtract 30 years from 1985, you know what happens in Back to the Future, datewise. 25 is not 30, people. Even for higher orders of 25.
Marketing ploy to sell the Blu-ray 25th anniversary edition coming out October 26, 2010? Indeed, that would be my guess. No one is more gullible than people on Twitter who retweet things that have a certain truthiness...and maybe have never seen the film. But those of us who have seen the greatest time travel movie of all time know better. They went back in time 30 years and then he went forward in time 30 years.
So if they put that rumor on Twitter every day until 2015, they will be correct. The great news is that, if "Back to the Future" lore holds up, in 5 years and a few months from now we will have retractable clothes and gravity-defying skateboards.
Bonus: The Family Guy parodies "Back to the Future" - in Italian!