I think you might want to take a closer look at your entire solution rather than continuing to do this or even make copies of the file.

The whole idea behind a time stamp is that it signifies when something was done. If you are going in and changing a timestamp then it means that the timestamp is, essentially, useless. I would recommend that you change your entire solution to some kind of persistent datastore like DBM or a relational database and then produce XML when, and if, it is actually necessary.

BTW: Your current solution is prone to disk failures and poor performance due to disk access/seek time issues.