The first accurate mechanical clocks were made in 1656 by Dutch Scientist Christian Huygens. His later models were accurate to within one second per day. If he set the clocks daily by the sundial he may have initially thought that the clock was varying in accuracy at different times of the year when in fact it was the clock which was accurate and the sundial which was varying.
We now know that all clocks are continually ‘out of step’ with the sundial because day lengths, measured by the Sun, increase by a few seconds over a period of about three months then decrease by similar amounts over the next three months only to repeat the process for the next half year. We can see what is actually happening on the next page.