title: When does a UNIX directory change its timestamp
body: <p>I used "touch" on a file, updating the file's timestamp but the parent directory's timestamp did not change.  However, (as expected) when I created a new file within the parent directory, the directory's timestamp did change.</p>
  
  <p>What criteria do UNIX-like operating systems (specifically AIX) use to determine when to update the timestamp of a directory?</p>