If you don't have a software editing program that can do it download Picasa (free) and then it can be done as either the watermark or as the "insert text" option. You select the photos you want it done to, add the date as above & then export the specific files to a new folder (many options as to resolution) & it's done while they are created in the new folder.
The time/date stamp on prints was an old gimmick on old film cameras. Nowadays, the time plus all the other details of the shot are just a part of the metadata. You can use post-processing softwaree to do whatever you want to do with it.