Thank you so much for your response! You encouraged me to go digging a little further. I had previously checked many photos and found no information in that top right yellow box, on the second line. Just dashes. And the date/time stamp listed under the Image number (name) was clearly wrong on many photos. So it looked like it only showed the import-into-iPhoto time as opposed to time the photo was taken.
However, when I just checked again, some of those very same photos did indeed have the correct time of day on them. I began to wonder if the time was changing based on when they popped in from Photo Stream. But I took a new photo and delayed the Photo Stream transfer, and it still showed the correct time later.
On further examination, I noticed I had some duplicates in my Photo Stream event. And one photo was correct time and the other was the later, wrong time. So, now I wonder how often (and why) I am getting these duplicates. These are not photos I purposely duplicated for editing or anything.
So I am very encouraged. Thank you for staying with me on this!
----------
I just noticed your previous question about where I imported my photos from, like maybe from an email. Many family photos come to me (from good phone cameras) via text or email, and I save them to iPhoto. Can I count on their data to remain accurate?