Any suggestion on freeware for windows that can do the exif
manipulations?

XnView does these things. xnview.com

Thanks to all who contributed. I am already using xnview but other than
being about to sort by exif date, I don't see any other exif related
feature. Perhaps I need to update to the latest version and try again. Or do
I need to download a plug-in?

I want to add that ipod touch (and therefore iphone, ipad) sorts photos
using the exif date. Therefore manipulating exif dates is the only way to
control sort order for ipod touch photo albums.

Here's more details:
http://support.apple.com/kb/HT4221

The problem that I'm facing (mixing photos from different cameras with non
synchronized time/date) should become more and more common as more people
learn to share travel photos.