The date stamp is relic of the old film days. In this digital age, it's considered as an ugly mark on the picture. All digital pictures saved in JPEG format will have the date included in the EXIF data embedded in the file itself. You can use most capable graphics programs to view that information.
A few old compact cameras still have it. There should be a menu item buried somewhere in the camera's menu system to enable the date stamp. If there is none, you can put the date there yourself using this.
Now you know!