Sorry about the confusion (and you are not the only one who got confused about this…). As stated in the online manual:
http://rayshobby.net/?page_id=730#program
If you only need to run the program once during a day, you can set a very large interval to ensure the program doesn’t run more than once.
That means the ‘Every’ field (i.e. the interval) should be a large number — in your case, putting 1 hour is large enough.
Technically the ‘Every’ field cannot be 0 because it will cause a divide by 0 error. The way the program works is it will start at the ‘start time’, repeat every interval time, and stop at the end time. As long as the interval time is large enough, the program will not run more than once, because the next time it’s supposed to run will be after the end time. Hope this makes sense.