Shared photo stream wrong / strange date

When adding photo's to a shared photo stream sometimes strange dates appear.

The problem happens with scanned negatives. In my libarary I sort all photo's on date, so I have changed the date of all these scanned negatives and applied them to the originals.

The photo's in my normal libary show the correct date and time, however when I put the photo's in the shared photo stream it all of a sudden changes to a different date...???

It might well be the scanned date that the negatives were actually scanned, but this orignial date is nowhere to be found in the metadata.

I have done some testing and it also does not matter what the actual creation, modification, and last opened date of the file is.

Anybody any idea where this original date is residing in the file, how can I change it, and why is the shared photostream taking this date instead of dates in the aperture library.

Thanks !!