portage currently unpacks tarballs which contains files with future timestamps quite happily, this leads to configure breaking in theese cases. I've had to do a find . -exec touch "{}" \; in two tarballs so far because of timezone differences between my system and the releasers packages.. admittedly, this is a minor issue, but still affecting at least me :)
Unfortunately, we can't fix this because often the relative settings of the timestamps are important and will affect the build process. :( Sometimes a find . -exec touch {} \; will mess things up as much as fix them, so these issues really do need to be addressed on a per-package basis.
