I was trying to be humorous. But I differ with you about a second. A 60th of a minute, and a minute is a 60th of an hour, the hour being a 24th of a day in which the Earth rotates once relative to the Sun’s position. So if the rotation of the Earth speeds up, so does a second. Minutely. Earthquakes, tides and wind all constantly alter the Earth’s rotational speed, but only in microseconds. Over long periods of time, it adds up but only as milliseconds. But a second can alter in duration.
No, the second is a physical unit, and hours and days are fixed multiples of it. To keep civil time from drifting away from meridian time, “leap seconds” are periodically added to calendar time, as needed.
This is an “intercalation”, neither the day preceding nor the day following is changed in length. The leap second is inserted between them.