Leap second

It's now 45 minutes past midnight by Greenwich time (UTC). My phone (on Verizon) has updated the leap second, as has the desktop computer, but the watch hasn't. When does the watch sync its clock with the phone?

Yes, Watch synchronises with your iPhone (and possibly has it's own NTP daemon, which _keeps_ it in sync), however, not every carrier provides NITZ (time synchronisation via cellular network signalling, usually once when the phone connects to the network, sometimes only when it's switched on), those that do probably won't have their own atomic clock, they use GPS (which is synchronised to atomic clocks).

Don't worry about it. The watch synchronizes time from your iPhone, which synchronizes time with your cellular carrier. They synchronize the time (in theory) with the atomic clock.

Your iPhone periodically syncs time from internet servers and then syncs the time to the watch.

Thus, your watch will get the updated, leap time when it next syncs with your phones after your phones has synced with internet time servers.

So, to clarify, on neither iPhone nor Apple watch will we be able to "see" 23:59:60 is that correct?

The adjustment isn't patched in advance, it will be adjusted through sync later?

I thought about this but I'm not so sure- my iPhone is synced with UTC which is currently 36s behind the Atomic Time. (TAI)

See www.timeanddate.com

Am still curious to observe the clock doing something new :)

Yes, all these automatic clocks sync with UTC which is always slightly out from Atomic Time. What happens with actual clocks which sync to a timekeeper such as Munich or Anthorn is that they start a sync at midnight and so it takes a few seconds to download the correction and apply it - in this case resulting in the second-hand pausing for 1 second at a few seconds part the hour. Your watch and phone may do this, or may not correct until 0100 depending on how their time check works.

Articles here about Atomic Time and UTC:

https://en.wikipedia.org/wiki/International_Atomic_Time

https://en.wikipedia.org/wiki/Coordinated_Universal_Time