Why the Apple Watch will be the most accurate way to ring in the New Year - Telegraph
www.telegraph.co.uk/technology/apple/watch/12074452/Why-the-Apple-Watch-will-be-the-most-accurate-way-to-ring-in-the-New-Year.html
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Lynch's team also used high-speed cameras to test the Apple Watch's accuracy, filming at 1,000 frames per second to create millisecond-precision. The watch face is then overlaid with the time from an atomic clock housed in Apple's testing lab in order to pair the Watch's time with the true time.