FWiW as an aside, it appears to count votes from December 2008 accounts (as just tested using this 2008-12-26 account).
So that's at least strictly before 2009.
So that's at least strictly before 2009.
If there's something magical going on, the ordering would be the only real distinction possible.
Apparently the original (still the same?) dates back to the OG PG days:
Ask HN: who knows about this - "http://News.YCombinator.com/classic"? - https://news.ycombinator.com/item?id=1832884
It's what the frontpage would look like if only the votes of users who joined in the first year counted. I made it to check if the site is declining. In practice it's usually just a slower-changing version of the frontpage.
~ pg