On the integration with Kiesel and Chrome, I'm pleased to see that engines/browsers can share the cost of developing new language features. Temporal is massive! Almost as big as the delta of introducing ES6. There are 4,000+ tests. The functionality does not need to be engine-specific, so it makes sense to leverage reuse.
I believe this is the first introduction of Rust into V8 itself. Which seems like a happy side-effect that hopefully makes it easier to share more Rust libraries in future. This helps keep browser development sustainable.
My only concern is that temporal_rs packages it's own time zone data, which may make the WASM package a little heavy, so I've been inclined to leave the polyfill up to fullcalendar's implementation.
Also, Diplomat supports traits and callbacks so you could actually make the timezone impl pluggable. Though we don't currently have JS support for that.
But also tz info isn't that big I think...