How can I set the modification time of a file on Mac OS X from the command line?
MacOS, and most (all?) Unix-like OSs have a touch command, that lets you modify access + mtimes.
on MacOS:
touch 06011200 file
Will set the mtime of 'file' to June 1st of this year.
GNU's version lets you specify times in a slightly more convenient format:
touch -d yesterday file