Dates changed on the photos
Exif dates are stored in your images as strings, including the year. So the fact that the phone was off shouldn't be relevant.

However it's quite common that image transfer programs re-write metadata, unfortunately.

Try emailing yourself one of the images from the phone directly and testing with that on another computer (or find some other way to get a file off the phone for testing.)

Switching the phone off won't wipe any Exif data. That's stored inside the files.