This seems to be a bug in Blogger right now. MarsEdit will let you change the date (see Post -> Edit Date) but It doesn't respect the changed time when I send the message to Blogger. I've opened a thread about this on the Blogger developer group at Google, and am waiting for a reply.
Covers your concerns, I'm definitely looking into solving this problem. I see now that you didn't explicitly request the ability to change the date, but I will lump that into the other request when I look into fixing the date problems with Blosxom.