I figgered out a quick and dirty little thing if you just want to backdate one day: Put this at the top of your post, say you're posting on Thursday, May 31:

<span class=date style="margin:0px;">Wednesday, May 30</span>

This will create a fake date header. When reading the posts, you can't really tell the difference. Make sure you use the same date format as is used in your site. I suppose you could use this any time, but the posts won't sort properly. You need the margin:0px because if the .date style in your stylesheet has margin:10px your "fakedate" will be indented. it does this because with .date now inside of the .posts block (with it's own 10px margin) you get a 20px margin.

Yeeeaaah dog!