True, but if someone is ordered to turn over 'all documents pertaining to topic X', they still might withhold documents. We don't allow the police to search all documents, just in case.
{edit} It would get messy if he was on a computer that had a faulty cmos battery (i.e. randomly your system clock is reset to the unix epoch). Trying to figure out the true date of files with timestamps of December 31, 1969 would be difficult. ;-) (I know that's prior to the epoch, but I had a faulty cmos battery in an old PowerBook and that's what would happen)
And they do use such limitations in cases where it'd be difficult/unlikely for them to be altered, such as timestamps kept by a third party. For example, subpoenas demanding that Google turn over email from a Gmail account usually specify a date range (I think there's even precedent that they're normally required to).
Except that timestamps can be altered.