The Earth turns 360° in 24 hours, so 15° per hour, 15′ per minute, and 15″ per second of time. At the equator one minute of longitude is one nautical mile, so 15″ of arc is a quarter of a mile — which means four seconds of clock error move you one mile east or west.
That is why the marine chronometer mattered so much. A clock that drifts by a few seconds a day is fine for a week; uncorrected for a month at sea, it can put you on the wrong side of a reef.
Today a radio time signal or any networked clock removes the problem, but the habit stays: note the time of every sight to the second, before anything else.