Complex sentence: Day length, or length of day, or length of daytime, refers to the time each day from the moment the upper limb of the sun's disk appears above the horizon during sunrise to the moment when the upper limb disappears below the horizon during sunset.
Split into simpler sentences: Day length, or length of day, or length of daytime, refers to the temporal length of a day, or 24 hours, during which there is daylight. The day length can be computed from the moment the upper limb of the sun's disk appears on the horizon during sunrise to the moment when the upper limb disappears at the horizon during sunset.