This question is often asked, and you don't find the answer in textbooks. If you have a group of clocks of one type, let us say that they are long-case ('grandfather') pendulum clocks, you might find that, once they have settled down and been set to the same time, they still all tell the same time within, say, a minute after running for a week. If you do the same with quartz watches you might find that they agree within ten seconds after a week. If you do this with several atomic clocks you'll find that they agree within ten millionths of a second after a week. It's reasonable then to say that the quartz watches are keeping better time than the grandfather clocks, and that the atomic clocks are about a million times more regular still. In the 1950s the regularity of the newly-invented atomic clocks was found to be better than that of the spinning Earth. In 1972 we started using atomic time for all our time signals.