There is a neat feature in Windows Live Writer (Version 2008 Build 12.0.1367.1128) that lets you set your post publish date to any point in time, past or future. This is very useful if you need to repost from an older blog to your new one and you want to retain the original posts date.
It might not work on all blog service providers. For example, Blogger doesn't support it and I get this dialog box. Check yours.