Photos export unmodified changes photo date and time

When you export photo from Photos, Photos will create a a new file, and this new file may have the file creation date the photo has been downloaded from iCloud or imported to Photos, when exporting as "unmodified original". The original capture date, when the photo has been taken, as set by the camera, will still be embedded in the EXIF tags of the exported image file, so the original capture date will not be lost. It is just not necessarily the file creation date of the exported image file.

Have you been using iCloud Photos to transfer the files? The download from iCloud is changing the file creation dates in Photos.

Because files are fungible, but the data is not.

It is like having the bank return the exact same dollar bill you deposited with them.

You will note that this complaint does not come up all that often, since most folks use the EXIF data, which is what Photos works with, instead of the actual file date.

It has been this way since Apple has introduced iCloud Photos. You may want to send feedback to Apple with a feature request to preserve the original file creation date. Feedback - Photos - Apple