I don't know how to do it in lightroom, but for placing geatag information i use geosetter (
http://www.geosetter.de" onclick="window.open(this.href);return false;) here you can also change the dates in the exif information, with a fixed time or a delta time. and you can do this for multiple images. After that you go to lightroom and select the changed images and press RMB -> metadata -> read metadata from files.
i hope this will do the trick