I have done some additional research on this and you need a 3rd party add-in in order to get real time on a working slide show. Once you open the "SLIDE MASTER" you will see the date/time block in the lower left corner. You can move that block to where you want it on the slide and resize it to fit your time and date needs. All date / time must be in that block if you want the updated during tie slide show.

Mike p