Last modified: 2011-08-13 18:23:28 UTC
The page [[Special:ListUsers]] has a option "Sort by creation date". I have not found a api param for this.
Maybe add a ausortcreation param to list=allusers.
There's no index on this, so this whole feature just sounds like a bad idea to me.
It appears as if [[special:listusers]] is actually sorting on user_id, not on user_registration (since user_id is auto-increment, earlier users have smaller user_id's in theory) which is indexed.
If you look closely at special:listusers sorted by creation date on enwikipedia, you'll notice that some of the users who don't have creation dates are interleaved with users who do have creation dates. (example: http://en.wikipedia.org/w/index.php?title=Special:ListUsers&offset=51&creationSort=1&limit=4 )