Bdtyktl

You can do this in PowerShell. Something like this to update the current folder based on files it directly contains:

$lastModified = (dir . | ?{!$_.PSIsContainer} | sort LastWriteTime | select -last 1).LastWriteTime

$folder = get-item .

$folder.LastWriteTime = $lastModified

(This will fail if there are no files in the folder or if the user does not have suitable permissions.)

EDIT: Ensure only one file (the last) is found so sort pipeline has a singular result.

I just did a simple test - if you create an empty file in the directory, it changes the folder modified date to to that time. When you delete the file, that still counts as a modification, so it stays modified at that time.

To do so recursively, you'd have to create an empty file in every folder you wish to have the date changed in. This is still a fairly simple solution if you're willing to do some simple programming.

If you want to change the date to something other than the current time, this solution obviously will not work.

You can use Bulk File Changer.

BulkFileChanger is a small utility
that allows you to create files list
from multiple folders, and then make
some action on them - Modify their
created/modified/accessed time, change
their file attribute (Read Only,
Hidden, System), run an executable
with these files as parameter, and
copy/cut paste into Explorer.

Not pretty, but functional. Done in bash / cygwin

==================

find -maxdepth 1 -type d | grep -v "^.$" | while read D ; do

  cd "$D"

  F=`ls -1tr | tail -1`

  cd ..

  touch -r "$D/$F" "$D"

done

==================

Brian

Here is a modified version of Richard's answer. It's also PowerShell script. Instead of setting the modified time of the current directory, it sets the modified time of all first-level subdirectories of the current directory. Another difference is that it looks recursively for the last modified time. Hope it's helpful for someone else.

$parentPath = Get-Location

Get-ChildItem | where {$_.PsIsContainer} | foreach {

    cd -LiteralPath $_.FullName

    $files = dir -Recurse . | where {-Not $_.PsIsContainer}

    if ($files.Count -eq 0) {

        echo "note: $($_.Name) contains no files"

    } else {

        $_.LastWriteTime = ($files | sort LastWriteTime | select -last 1).LastWriteTime

    }

}

cd -LiteralPath $parentPath

Do Nothing! Mission accomplished!

The last modified date of a folder is always that of the last modified file in it.

FolderTimeUpdate

Really good choice for non-scripting program. Aside from not being configurably automatic (in the GUI), its functionality is exactly that desired:

FolderTimeUpdate is a simple tool for Windows that scans all files and
folders under the base folder you choose, and updates the 'Modified
Time' of every folder according the latest modified time of the files
stored in it.

Most of this can be accomplished on the command line as well, so conceivably you could automate it by setting it to run at intervals, or perhaps even monitor certain folders.

The GUI allows control of subfolder depth to recurse, inclusion and exclusion wildcards, and even whether to change the file creation time also (same as modified time, or match oldest creation or modified time).

Program by the excellent and prolific Nir Sofer (the same author of Bulk File Changer in a less relevant answer).