A solar day is the interval between two noons. It is not 86,400 seconds exactly — the Earth's orbit is an ellipse, the axis is tilted, and the equation of time makes sundials argue with clocks. Atomic time ignores all of that, then we insert leap seconds so civil midnight stays near the Sun.
At the equator, day and night stay near twelve hours all year. At 78° north, the Sun can refuse to set for months. Agriculture, prayer, and sleep were all first scheduled against this local light.
Move the observer and the same date becomes a different day.