Roger Olsen has an interesting post wondering why calvinists don't think it limits God if he cannot change the past. But both Olsen and his calvinist sparring partners are wrong, God changes the past all the time, it is just no one ever notices because he changes our memories too!
However, there are some tell tale signs that this goes on more than we realise. For example, thousands of years ago when Moses wrote Genesis the world had only taken 6 days to make, but today if you examine the planet and its surrounding environs you can see it took around 40 Billion years to make. Or if you read ancient documents you would see that the Sun used to rise up in the sky and go down at night, but today it is Earth which rolls around the Sun. The only possible explanation is that what used to be really was then, but then it got changed retrospectively and now we are living with a new version of the past. QED.
Of course the nice thing about having a changeable past is that when we argue about what really happened back then, we can both be right! And as far as I remember I never lent you that $20.