What is layer 1? What does this have to do with the iPhone or the Apple Watch?

You may have heard that reference when talking about the Apple Watch or iPhone or iPad. Although it's also possible that you haven't felt it in your life ...

Apple uses 15 Stratum Tier 1 network time servers around the world, so its connected devices offer a fully reliable time.

Each server is connected to GPS satellites orbiting the Earth, which in turn receive the time from an atomic clock located at the United States Naval Observatory.

Apple has included fixes in the software to compensate for latency (the time it takes for the signal to travel from sender to recipient) and to compensate for communication delays.

So, if you put two Apple Watches next to each other, their hands will move perfectly in sync.

As they explain in calculable thoughts:

There are several types of servers of this type. The ones with the highest level of accuracy are stratum-0 (stratum-0 in Spanish), which are devices that include atomic clocks.

These clocks are based on the oscillation of the cesium 133 atom, which measures one second every 9.192.631.770 oscillations and has a one second drift every 300.000 years (drift is the time that deviates from the perfect clock), so these are the world reference watches.

Within this group of strato-0 servers there are some that have a slightly lower level of accuracy, as they receive the time via satellites (using a GPS) or radio stations and correct it taking into account the transmission time and the Einstein's theory of relativity.

In radio transmissions the error can vary from 0,1 to 10 milliseconds, while for the satellite the error is about one microsecond.

No computers can be connected to the stratum-0s on the network and they send the time to the next server type, the stratum-1, via a direct cable. We can connect to layer-1 over the network and they are the main time servers on the internet. 

Source: Stratum: How do we know what time it is? - NTP server