keynote slides

At a certain time of day? or at a certain time on screen? it isn't clear from your post.

The latter is easy, using a timed transition on the slide to trigger after a certain time (not sure if '12:00' means 12 hours or 12 minutes...

If the former, however, there is nothing in Keynote itself that directly ties to the system clock - you can't advance a slide at a specific wall clock time. To do this you'd need some external trigger such as an AppleScript with either an idle timer, or a launchd or cron action to fire the script at the appropriate time.