For a Canon Powershot S51S

You want to go to the SETUP menu and select the PRINT menu. Then select the PRINT SETTINGS. Then choose the SET button. You can now use the arrow buttons to scroll up and down until you see the DATE option. This is where you can set this option to ON (which will add the date to the photos you want to print).

You can see the exact instructions on page 137 of the User's Manual here from the Canon website. http://gdlp01.c-wss.com/gds/0900009893/PowerShotS5IS_CUGad_EN.pdf

Have you set the date and time? If you have set them, the information of date and time will be stored in the file of the photo.