The mtime (modification time) on the directory itself changes when a file or a subdirectory is added, removed or renamed.
Modifying the contents of a file within the directory does not change the directory itself, nor does updating the modified times of a file or a subdirectory. Additionally, adding, removing or renaming files/directories in subdirectories does not propagate up to the directory. If you change the permissions on the directory, the ctime changes but the mtime does not.