UTC offset for an IANA timezone at a UTC instant, minutes east, from the
host's Intl: a cross-check on the host's time-zone data. zoneOffsetAt is the
engine's own answer, the one its wall times use; the two agree from 1970
where the host has tzdata 2025c, and before 1970 they can differ, because
Intl does not carry the history this package ships.
UTC offset for an IANA timezone at a UTC instant, minutes east, from the host's Intl: a cross-check on the host's time-zone data. zoneOffsetAt is the engine's own answer, the one its wall times use; the two agree from 1970 where the host has tzdata 2025c, and before 1970 they can differ, because Intl does not carry the history this package ships.