See page 33 of your User Guide (note there are two user's guides... a basic guide and an advanced guide. This is on page 33 of the 'advanced' guide ... the full instructions rather than the abbreviated version of the manual.)
Here's the link in case you no longer have your user's guide: http://gdlp01.c-wss.com/gds/5/0900001215/02/PSA570ISCUGad_ENg.pdf
In short, press the menu button... the screen will show two categories or "tabs" at the top... you want the tab with the camera icon. Scroll down until you find the words "Date Stamp". This defaults to "off" but you can "Date" (only the date, not the time) or "Date & Time".