I have two digital cameras, one Panasonic and one Casio. Both have "DPOF" option to put time stamp on pictures. I specify "on" to print the date on the pictures, but to no avail. What do I miss?

The DPOF option does not enable you to stamp the time and date alone. DPOF is an abbrevation of Digital Print Order Format. A DPOF-compliant camera can create a print order file that describes which shots should be printed, the quantity of prints desired, which thumbnails should be output as index prints, cropping and rotation settings, and user and picture information (names, descriptions, date stamp, and so on). The DPOF file is stored on the removable memory card, which allows you to either stick the card directly in a DPOF-compatible printer or send the files to a digital printing service.
You might have to make certain print settings within the camera. If you have something similar to "Print Order" or "Print Settings" in your menu options, the sub menu will have a "Date" option. Make sure that is set to On. Not the date option in Display.

You might want to rethink the whole date stamp thing. If you are using a digital camera then the date will be accessible when you upload your images to your computer. You should only have to either click or right click to find out the date. I used to like the date stamping but when I would have a really good picture I wanted a 5x7 or 8x10 the date would look corny and greatly take away from the picture. I was so excited when I discovered digital and I will NEVER use stamping again!

You can add the date in the file name of the photos. This can be done with a free batch process program such as File List for Mac or Batch File Renamer for WIndows since sometimes when files are copied the encoded info with the creation date is reset. I've seen this many times but putting the date in the file name you can't go wrong. Example: original file is DSC123456.jpg and after adding date DSC123456-7-31-08.jpg. Of course there's Photoshop's professional digital invisible watermark system using Digimarc but service is expensive to register for. Hope this helps.

You can use a tool to add date to photos. Try this: BatchStamper --batch add text and date to your photos