When does a UNIX directory change its timestamp

Question

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.

What criteria do UNIX-like operating systems (specifically AIX) use to determine when to update the timestamp of a directory?

Answer 1

The timestamp is updated when the data that represents the directory changes. A change in a subdirectory of directory D does not change anything in the representation of D because D only points to the subdirectory, not to what's inside it. On the other hand, creating a file in D changes the block of data on disk that represents D.