With chronometer-rated mechanical watches now boasting movements accurate to a couple of seconds a day, one might have thought the eons-old battle for timekeeping precision was well won. Not so. Smart watches pull in their time signals from satellites, making accuracy a challenge within fractions of a second.

Safe in such knowledge, I'd assumed my two smart watches would display the same time. But I recently noticed that they differed, as did the phones they were tethered to. While my iPhone and Apple watch tallied, my Android twins, a Samsung Galaxy and TAG Heuer Connected, were five and seven seconds ahead respectively. Which display was right?