I think what you are looking at is the watering duration for the instant manual run (not the user program) in the zone sections shown on the home page. The system remembers the last duration of the instant manual run and displays that on the home page. The watering time for the program remains unchanged. You can go to Settings->Programs or Settings->Schedules\ to verify.
Please tap on the "15:00" text and there will be a window pop up for changing that value.