Why Do My Photos Show No Date at All?
A blank Date taken field means the photo has no EXIF date inside it. That is normal for Takeout exports, and it is fixable.
A blank date is not corruption. It means the photo genuinely has no capture date inside it, and your viewer is being honest about that rather than guessing.
Photos get their date from the EXIF block written by the camera at the moment of capture. Plenty of files never had one. Screenshots have no EXIF date. Images saved from WhatsApp, Messenger or a browser usually have it stripped. Anything you edited in Google Photos lost it. And scans of old prints only have a date if someone typed one in. For all of these, Google Photos still knew the right date, because it stored one in its own database. Your export contains that date in the .json sidecar, not in the picture.
So the information exists. It is just in the wrong file.
The app copies it across. It reads photoTakenTime from each sidecar, writes it into the photo as a proper EXIF date, and sets the file timestamps to match, so the blank column fills in everywhere at once. Where a photo has no sidecar either, it falls back to the date in the filename, and it lists anything it genuinely could not date so you know what is left. The first 500 photos are free.