Thanks for the detailed response — I already had the feeling that there must be some kind of "it's groundwork for the next WWDC"-motivation here ;-)
But as usual, answers just produce new questions...
Not sure how to understand this… I guess "composable" refers to the problem that transmission takes time, so that your actual deadline is always slightly bigger than intended, right?
That obviously wouldn't be nice, but it's impossible to get two clocks in perfect sync (at least usual clocks), so there will always be some difference.
I just tried to find some common values, but only found vague answers ("should be better than 100ms"), but then I remembered I had to tweak the configuration of an NTP-client in the past, because it was sending emails every day… so, from live experience, I can say there is not a single day in the logs where the needed adjustment was smaller than 1.2 seconds(!).
I'm not sure what a typical timeout will be, but I'm sure you cannot guarantee that the difference of two computers clocks is small enough so that you can express timeouts correctly using absolute time.
So are you going to have an additional clock which will deliver wall clock time plus some delta to compensate differences? But if you do that, you could also use MonotonicClock, couldn't you??
The "two computers" scenario also made me think that WallClock might not be a good name: When you call someone in Australia and ask what time their wall clock is displaying, you'll end up with a difference bigger than any common timeout ;-); maybe WorldClock would be a better fit?