The sun is setting behind the misty hills on this Spring equinox. Although “equinox” means equal day and night, the day was not actually 12 hours long today. That was three days ago when the day and night were equal at the “equilux.”
At the equinox, the measurement is taken from when the centre of the Sun is at the horizon and there are12 hours between sunrise and sunset.   The top edge of the Sun is visible first at sunrise and last at sunset. Due to light refraction, there is still light for a short time even when the sun is no longer visible. This adds a little extra time to daylight hours.