I'm new to JabRef but as I started I found I could not do the first thing I wanted. When importing my 1000+ XML-based (from MS Word) references (mostly IEEE), I found that all the references imported to JabRef had the month field empty!
I did this test and found that this is a problem when importing from *any* XML file, even if created by JabRef irself. You can just create a reference in JabRef containing a month, export it to XML, and import it back. It won't read the month field that was initially there.
I believe this is a bug. A friend who recommended JabRef to me tried this and was surprised to see it happening in his computer as well. We´re unable to find any support for this in the internet.