Make the Type tool active and enter the text.
If you have several photos that you want to add the date to, in full editor, go to File>Process multiple files.
In the dialog under "Labels", select "Watermark" from the drop-down, enter text/date in text box, then set up the other fields.
It's best to have the picture files in a source folder & have an empty destination folder to point to in the batch process. This will preclude overwriting the originals. Look at the top "Process Files From".