LR 3 and 4 are buggy when handling images whose metadata don't have capture dates (EXIF:DateTimeOriginal). For such files, the date/time shown next to the Library thumbnails is not the capture date -- it is the date-created of the file (as set by Windows or Mac OS).
In order to get a capture date stored in the original file or in exported versions, you'll need to give the command Medata > Edit Capture Time. Often, all you need to do is select all the photos that don't have a capture time, do Edit Capture Time, and then click Change All, and the photos will now have their capture time set to the time the file's last-modified time. However, I strongly recommend that you take a backup of these photos first, since Edit Capture Time can't be undone within LR.
See this feedback topic for more information:
John - very helpful information, thanks!
I've been doing what you suggested (Edit Capture Time), and sometimes the file created time is messed up for some reason (like, the year is shifted by +1 or something) so I've been also using ABF Attributes to correct the date created then set capture to it.
Your explanation was life saving - tx!