Function wall_clock_ns 

pub fn wall_clock_ns() -> u64

Wall-clock nanosecond timestamp (Unix epoch).

Use for: user-visible timestamps, mutation attribution, cron emission. Never use for: ordering or durations within the dispatcher (use monotonic_ns — wall-clock can jump backwards on NTP correction).

Returns 0 for pre-epoch system clocks (misconfiguration). Saturates at u64::MAX past ~2554 — same reasoning as monotonic_ns.