The file time is not necessarily the time the photo has been taken. If you upload the file it gets usually a new date, the one of the upload (since it is a copy technically). The date taken would be embedded in the file's metadata.
Also a date like 13.01.2013 is not really sortable since it is no value. To properly sort by date you might need to use either the international date format yyyy-mm-dd or better a unix time stamp since that provides a real increasing number.