In an interview with The Telegraph, Kevin Lynch explains how Apple Watch tells time:
"The second hand on every Apple Watch is perfectly in sync, and each device is accurate to 50 milliseconds of Co-ordinated Universal Time (UTC), the world’s time standard, which is below human perception, so when you look at it you can’t tell the difference," he explains.
Apple built its own Network Time Protocol (NTP) servers at various locations to ensure the delivered time is as close as possible to Stratum One accuracy, the time server which keeps the Apple Watch within microseconds of Stratum Zero devices - the highest possible quality for time references.
Once the time reaches the Apple Watch, the team worked to ensure it remains accurate, he says. Each device uses a temperate controlled crystal oscillator to counteract the natural drift that clocks and watches tend to experience over time.