The manual sholud tell that.
Outting date time in an image as a 'desktop' function is an interesting proposition.
Dont know if such a software exosist-a Google might reveal somethine.
The date time info is in every image taken and can be revealed my finding out how to reveal the EXIF info when the image is displayed in your image editor.
Irfanview will do it so will PSP X
Photoshop should therefore- seek a bit of PhotoShop online help if you cannot find it.
PSPX which is a close relative to PhotoShop reveals this info by the following;
Load an image/top tool bar/image/image
Now to discover how to use this information in Photo-shop.
Over to you