(late) reply to your answer dating December 17, 2011 Rated 4 out of 5 stars

sorry for my late reply to your answer.
No, the "add date to the file properties" feature does not work properly yet.
As you know, each file (in Windows) has three timestamps: 1) created; 2) modified; 3) accessed.
Whenever you see the detailed list of files in Windows, the timestamp shown is the second one (modified), and you can see the other two by cliccking on "properties". Also in WinZip the timestamp shown is the second one.
If I am not wrong, your add-on sets the third timestamp (accessed) and lets the other two fields empty. So, sometimes Windows adds the current time to such fields. I would suggest you to set the timestamp of the second field (modified) when the feature "add timestamp" is thicked.
I guess that the problem is connected with new versions of Windows, for I am experiencing similar problems with the timestamps of pictures taken with my digital cameras. If I download them on a Windows XP computer the timestamp is ok, if I download them on a Windows 7 or Vista computer, I get the timestamp of the moment in which I downloaded them, but some programs are able to correct the problem (Olympus software is not able, Canon software is).

This review is for a previous version of the add-on (2.7.2.2). 

Rated 5 out of 5 stars

I really like this extension, and had been looking for it for a while.
There are still some problems with the timestamp, but I fear it depends on Windows 7 rather than on your extension. Sometimes, not always, you have problems with the timestamp in these cases: 1) if you add a prefix - by chosing the "add a refix" feature - to the filename of the file to be exported; 2) if you export new messages to a folder in which there are already exported EML files: it may happen, but does not always happen, that th previously exported files change their timestamp to the current day and time (as it happens with Windows 7 when you copy/move an EML file)

This review is for a previous version of the add-on (2.6.3.1). 

maybe it's already there?

I don't know if I've understood your question (I've not win7), but maybe the feature you ask is already there: go in extension options panel and enable "Add date to file properties" (yes, the label is too cryptic!!!). Did you mean this?