Just a quick note for myself, that I hope others will find useful.
To set the modified date of a file in OS X to a value in the future (useful if you want something to always be sorted at the top or bottom of a date listing), use this command in the Terminal:
touch -mt 201212120000 [path to file]
The date format is YYYYMMDDHHMM (Year-Month-Day-Hour-Minute), so in my example above, I’ve set the modified date to midnight, 12/12/12 (December 12, 2012).
Bonus tip: if you don’t want to type the full path to the file, type
touch -mt 201212120000 and then drag the file onto the terminal, which should write out the path for you.
Bonus tip #2: the command will work on any *NIX system, though I don’t expect the same holds true for the drag-and-drop file path.