In the version history of an image (or any attached file in MediaWiki), the page displays "Date/Time" with a link to that version. The timestamp displayed is the upload timestamp of that version. If you look closely, you can see that the real filename includes a different timestamp. This turns out to be the timestamp of when that file was superseded by a subsequent version.
I have looked in the database tables and can see that in the oldimage table, each row has an "oi_archive_name" with the timestamp of when that version was superseded and an "oi_timestamp" of when that version was actually uploaded.
Is there a reason to name the old versions of the files with the superseding timestamp instead of the upload timestamp? It seems to me that the timestamp of when that version was uploaded is more relevant.
Daren