This is due to Picasa 3.9 stripping EXIF data from photos when uploading to Picasaweb. If you look at the 'Full Details' information for a photo uploaded with Picasa 3.9 you'll see that most EXIF data, including the Date and Time, is missing. The only workaround I found was to use the use the Picasaweb online uploader to upload photos.
The problem is supposed to be fixed now. See this thread: http://productforums.google.com/forum/# ... Hk1JMAQF0E