Photo Date Stamper is a free program which can watermark a batch of photos with the date they were taken.

You're able to configure the watermark by setting its font size, colour and position (top left, top right, bottom left or bottom right). There doesn't seem to be any way to adjust the date format.

It's possible to add a short description which appears under the date. A Preview button shows you how this will look.

When you're ready, click Run and the photos are watermarked and saved to your preferred destination folder.

If the files don't have a "date taken" tag then you'll be prompted to enter a date manually. This can be applied to every untagged image, or you can enter a new date for each one.