I think the title says it all.
My first post is dated 2010-08-10 00:24:47Z.
I received the yearling badge at 2011-08-11 00:16:33Z.
Shouldn't this have been awarded 2011-08-10?
It's not based on first post, it's based on the time of account creation.
Mouse over the date in the "member for" field to see the UTC time your account was created.