Sometimes I come across an old photo where the date and time stamp are all wrong, and then looking at the EXIF data, there's nothing there. It seems this happens mostly with old panographs where the software does not copy EXIF from source to destination.
EXIFTool can fix this for you:
exiftool -tagsfromfile source.jpg -all:all destination.jpg
The original is rename to .original, you can then use jhead to fix up date and time.