Function moment_add_span 

pub fn moment_add_span(nanos_since_epoch: i64, months: i32, days: i32) -> i64

Add a calendar span (months then days) to a SmoothUTC instant — the civil (wall-clock) operation: months clamp at end-of-month and respect leap years, the time-of-day rides along untouched. Distinct from adding a physical Duration (which is just nanosecond arithmetic). Delegates to the same base::temporal::add_span the AOT mirror uses, so the tiers cannot diverge.