Barb B - I just dragged the epub you sent into calibre, ticked *only* the update metadata option in Modify ePub, and it all works perfectly fine.
As you say in the ePub the date field in that metadata looks a bit screwed up. I would suspect that this comes somehow from the metadata associated with the book as it exists in your calibre library. In my updated ePub this has:
Code:
<dc:date>2011-09-26T14:00:00+00:00</dc:date>
This plugin is calling standard calibre code for this feature. So I would expect you to get exactly the same result if you were to use the Save to Disk feature of calibre, where it applies the same updating of metadata.
Try that save to disk and see if you get the result I predicted. Also make sure you are only checking the Update metadata option, in case in fact it is some other option that is somehow disrupting the data.