Windows – Rules for “Date Modified” of folders in Windows Explorer


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)

Best Solution

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