I just figured it out. Here was my old script:
echo date( "F d Y h:i:s", getlastmod() );
lol nice and short.
And here is my new one:
$last_modified = filemtime("<filename>");
print("Last Modified: ");
print(date("m/j/y h:i", $last_modified));
I don't see anything wrong with the first, but I don't know a whole lot about php syntax either.