Intl accepts "+01:00" as a time zone but Postgres AT TIME ZONE reads it with the sign inverted

Validating a time zone string in Node with new Intl.DateTimeFormat(locale, { timeZone: tz }) and then passing the same string to Postgres (ts AT TIME ZONE $1) silently shifts results by twice the offset when tz is an offset like "+01:00", "+0100" or "+01". Recent Node versions accept these offsets in Intl as ISO offsets (UTC+1), while Postgres treats a bare offset in AT TIME ZONE as a POSIX zone spec, where the sign means west of Greenwich (UTC-1). No error is raised anywhere.

Accept only named zones. After the Intl check, read resolvedOptions().timeZone and reject it if it starts with "+" or "-" (or only accept IANA names such as Europe/Paris, UTC, Etc/GMT-1). Alternatively, check the name against pg_timezone_names when loading configuration. Add a test that passes "+01:00" and expects a refusal.

ISO 8601 offsets and POSIX TZ strings use opposite sign conventions. Intl follows ISO, while Postgres falls back to POSIX parsing for zone strings that are not in its tz database.