StocksAndFisheriesKB

In the example you provided http://firms.fao.org/fishery/xml/resource/10023/en

there is the creation date:

fi:CoverPage
dcterms:Created2014-05-21/dcterms:Created
/fi:CoverPage

The modification date is not mandatory, can be available or not. If it is available then it is the date to consider to fill the field "Updated [Update date]", otherwise the creation date is the update date.

if you check the fact sheet in its source website http://firms.fao.org/firms/resource/10023/en the citation reads:

© FAO 2014-2019.
ICCAT Standing Committee on Research and Statistics (ICCAT- SCRS). Stock status report 2013. Swordfish - North Atlantic. FIRMS Reports. In: Fisheries and Resources Monitoring System (FIRMS) [online]. Rome. Updated 21 May 2014. [Cited 28 March 2019]. http://firms.fao.org/firms/resource/10023/en

Finally, if the creation date is missing, then it is an error of the source data which has to be corrected in the source database and the GRSF citation will have that field empty until it will be fixed.
Hope I am clear, we can have a call on this if you want, sorry for this burden, but indeed there are many standards and guidelines on this matter.

Thanks Aureliano,

Perfectly clear. I'll proceed as you suggested.