Here's how to change a file's modified date:
touch -mt 201102260000 <file>
Where "201102260000" is year, month, day, hours, minutes all glued together (so 2011-02-26 at 00:00).
Also, if you want to change both the modified and creation date, then:
touch -t 201102260000 <file>
This works on OSX and (i'm assuming, since touch also exists there) Linux/Unix.