I don't think you posted enough information about your problem here.
Is the data being stored in a database or in some other format (like flatfiles)?
Do you ask the user to put their username into a field when they post?
You can always get the date by using the PHP date() function. Try this:
$postDate = date('Y-m-d');
If you have the username as a posted variable (say $user) then just tack this string to the data record:
"[edited by $userName at $postDate]";