Export from photo library loses date in half the photos

Question

I used a separate iPhoto library to pare down duplicates from a folder of assorted photos that I eventually wanted to integrate into my main iPhoto library. All 3000+ photos had the correct date. Approximately half of these survived the purge, and were exported (full size, with file name included) to a file folder. When I imported these photos into my main library, I discovered that approximate half of these now had today's date. Inspecting the folder they were imported from, I found that the creation date and modification date of these 750 or so photos were both today's date, rather than the date they had had when in the separate small iPhoto library.
What would cause just some of the photos to lose their original date?
(As a work-around, I have now imported the original 3000+ photos into my main library, where they show the correct original date. I have further given them all a unique keyword, and created a smart folder for them so that I can continue to import other photos while I work to once again remove the unwanted duplicates from this set.)

Answer 1

I had chosen JPEG, but I subsequently ran the same export using "Original" and then "Current". The last two exports changed the dates in about 95% of the images to the current date. The JPEG export changed only about half of the dates.
I seem to have had a problem with some of the subject images being corrupt where the Information panel showed the Kind as "_" in iPhoto, but the Extended Photo Info still showed the correct date in the File Modified field. This field then got changed to the current date when the file was exported, regardless of which export "Kind" was chosen. This changed date was consistent viewing the image in the finder or in iPhoto, either in the Information panel or in the "Extended Photo Info" panel, and was the date used by iPhoto for sorting the photos.
Not very useful for hunting down duplicate photos.
So I imported ALL the 3000 or so photos directly into my main iPhoto file, where they kept their original modification date, and then spent the last day or so comparing them to my main database of about 13000 photos.
So why does the Export feature modify the date to the current date? I know you specify that the file data and the exif data are different beasts, but I don't see any place that they differ.