datetime - Rules for "Date Modified" of folders in Windows Explorer

Question

How does Windows Explorer determine the "Date Modified" field for folders? [Aside: I know this is asking from an explorer-specific perspective, but the behaviour could be useful to coding search/sort type activities]

Is there a definitive description of this anywhere - searches of Microsoft, MSDN, Google & Stack Overflow have been unsuccessful.

Personal experiments seem to suggest that in a tree of folders:

1. when a folder/file is added/deleted in a folder, the containing folder's date modified is updated.
2. when the content of a file is modified, the containing folder is unaffected

However, I'm looking at some directories that don't seem to conform to these basic rules - running down the tree I have (dates in DD/MM/YYYY format):

folder     (date created:  2/2/2006, date modified: 2/3/2006)
  folder   (date created:  2/2/2006, date modified: 2/3/2006)
    folder (date created:  2/2/2006, date modified: 1/6/2009)
      file (date created: 27/3/2009, date modified: 2/2/2006)

Answer 1

Disclaimer: I tested all of these myself on Windows 10. I could not find an authoritative source documenting all of these behaviours. It is entirely possible that I made a mistake somewhere.

The folder's last modified time is updated for these actions:

• new file or folder directly in target folder
• renamed file or folder directly in target folder
• deleted file or folder directly in target folder
• hardlink create/delete/rename - same as files
• file/folder symlink create/delete/rename
• directory junction create/delete/rename

It is not updated for these actions:

• modified contents of file directly in target folder
• edit target of symlink or junction contained in target folder
• file's or sub-folder's created/modified date changing
• edit basic attributes (hidden/archive/system) of a direct child
• NTFS compression/encryption change of a direct child
• anything at all happening in a sub-folder - literally anything
• changing attributes of the folder itself
• changing owner/ACL of the folder itself
• owner or ACL of a direct child changing
• if the folder is a directory junction, changing the target
• adding/deleting alt data streams to a direct child file