Outlook Express. change the date on sent messages

It can be done, the caveat is, it will take time.

Drag a message out of Outlook Express onto the Desktop or into an empty folder.
Right click on the message,
Open with...
Select Notepad

I have a shortcut for Notepad in my Send To folder, all I have to do is right click on the file and choose Notepad. The message opens in Notepad.

The message headers will appear. In the message header there are about several date locations.
Correct the dates then save the file, File > Save
Close the file, then drag it back into Outlook Express, if all goes well, the corrected date should now appear for the message inside Outlook Express.
Repeat for subsequent messages.

I would keep that "wrongly dated sent emails folder" to itself (as the master copies) and drag the edited messages into another folder in Outlook Express, in case of unexpected or undesired results. You can then start over if needed.