I've noticed quite often that what an album's release date is on the archives and what it was on wikipedia are quite different. Spare me the wikipedia can be edited by any argument, because in this particular case, the archives Nightwish's Oceanborn was released on October 1st 1998, where as wikipedia says it was released on December 7th 1998, and the latter is backed up by the band's website as a source.

http://nightwish.com/en/news/1998/12/oceanborn-is-released

What I want to know is where the mods (or the editor) got the idea that Oceanborn was released on October 1st? (Particularly since the recording data indicates it was still being recorded in October)

Sadly enough, this happens quite often when old albums get released digitally (displaying a "rough" release date).
https://itunes.apple.com/us/album/sleep ... =279236251