Aah - I don't think there is any way to selectively pull metadata elements from the format files, its all or nothing.
Are you interested in the pubdate of the edition you have, or the pubdate of the original, if the latter then I don't know of anyway to download or extract, I cut and paste from the viewer - if its there?
You could set the bulk metadata edit download off to just get pubdate from one source and let it run overnight. The metadata download speed is choked, this is to prevent calibre users overloading the source providers servers. If calibre didn't do that then they would complain to Kovid. They provide the information for free with no ad revenue, so they deserve a fair shake of the sauce bottle
BR