Most likely that the online sources are wrong, since the quote I linked to is by Steven Schend, a longtime Realms designer and friend of Ed's.and now that the 7 to 5 dates are off what does that mean?
Zirta is mentioned in Grand History of the Realms, if I recall correctly, but I don't have my books (they're all in storage) so I can't check it to see if a date is listed.