Because every day does not have a set number of daylight hrs. Some months sun gos down at 6:30p others at 8:30p. I would like to see a "Middle of the Day watering" option. Simple Sunup time minus Sunset time (basesed off astro) divided by 2 . System waters the users set amount per zone consistently at actual mid daylight no matter the number of hours of sunlight at your location. I see people using this as a backup for extreme heat days. Or when new plants or sod are planted to allow best chance for success.