Day length and solar noon
Day length is the duration the Sun is above the standard horizon within the selected civil day. Solar noon is its upper meridian transit, not necessarily 12:00 on the clock. Longitude, time-zone rules and the equation of time affect the clock reading.
Polar day and polar night
At high latitudes the Sun can remain above the horizon without rising or setting that day. In polar night it can remain below the horizon while twilight still occurs. No sunrise does not necessarily mean complete darkness throughout the day.
Daylight-saving transitions
A local civil date can span 23 or 25 hours when clocks change. This calculator finds the actual beginning and end of that date instead of assuming a fixed 24-hour UTC window. The resulting daylight duration uses elapsed time.