You can edit the photo in the Photos app, choose the pen tool and scribble anything you like on the photo. If you want to be a bit neater you can use an app like Annotable to place text on top of the photo.
It should also be possible to create a shortcut in the Shortcuts app to put a date and or time stamp on the photo. I'm not completely sure how to do that. It probably involves the Mask Image action and a few other tricks. Or you could just create an image with text and combine it to the bottom of the original.