Ignore annote field if it's exort date

We're working for a university thtat needs some improvements on BibText modules. They download info from zotero and some of the fields ara not beeeing imported correctly, so we'll be doing some features to expand the bibtex module. We will be keeping issues separatly so you can approve any of them.

On this specific issue, we have detected that some inproceedings were being exported with some annote indicating the time of the export, that ment that the important annote were deleted, so the program only save the last.

The date that the info was exported is'nt really necessary, so with this patch it will be ignored.