With the Temporal API will all of the browsers / nodejs etc be able to get the time right when performing operations around that time or will they need to be updated in order to not mess up calculations?
Most datetime APIs are fundamentally designed and intended for supporting calendar and wall clock operations for business functions. If you need SI seconds for scientific purposes, you really need to use alternative APIs and facilities that provide and guarantee the semantics required all the way down to the hardware level. Likewise, if you want timers, etc, for software facilities like thread sleeping, you use dedicated interfaces like monotonic clocks. If leap seconds are phased out, this won't really change the situation. It was wrong for software to rely on Unix timestamps for, e.g., mutex algorithms before and it'll be wrong if and when leap second clock adjustments are gone.