It is really easy for the Police to change the date on a file. The most corrupt police force in the world (the UK) did this to a persons hard drive in an attempt to prove that he wrote a letter to a person on a date he wasn't in the country. It would have worked too except that the "suspect" had only purchased the hard drive AFTER he returned. This was a few years ago and so they had no case.

It is so bad in the UK now that he would have been arrested, charged and convicted irrespective of the fact that he could prove he wasn't in the country!