@efx: This can't be done automatically, however you can manually add the date to an image by heading to the Gallery app > Select an image > Tap the pencil at the bottom of the screen > Tap the sticker icon (square smiley with the bottom right corner turned upwards) > Tap the clock or calendar symbol > Choose your preferred sticker. The date the photo was taken will automatically populate, so just drag and drop it into position, then hit Save at the top of the screen.
