Start time, stop time, and interval time together determine how many times a program runs during a day. Keep in mind that a program can run more than once during a day. The firmware user manual explains how the controller runs a program (http://rayshobby.net/?page_id=730#program)
For example, a program that ‘start at 8am, water every 4 hours, and stop after 4pm’ will run at 8am, noon, 4pm.
I admit the end time is confusing, so there is already plan to change the program parameters to ‘start time, repeat every, and repeat count’. This way it’s easier to set how many times the program runs during a day.