Function default_epoch_us 

pub fn default_epoch_us() -> u64

RFC-0052 W3b.2 — the default wall-clock source: µs since the UNIX epoch.

phase-359 W10 — this was std_epoch_us, std-gated, SystemTime. It is the same story the monotonic clock had one commit ago: rmw-cffi means a platform port is linked, every port exports the wall clock, so a hosted build reads the same source an embedded one does. SystemTime survives only where there is no port to ask. The claim that stood here — that the metadata probe is that configuration — was WRONG; the probe links a port and resolves rmw-cffi. See default_clock_us_fn for what was measured.