Function system_timezone 

pub fn system_timezone() -> &'static Tz

The OS-configured IANA timezone, read dynamically. Uses iana-time-zone (multithread-safe on Unix, unlike time::UtcOffset::current_local_offset, which errors in a threaded process). Falls back to UTC with a warn only when the zone can’t be resolved - the rare exception, so “today” tracks the user’s wall clock.