Using the upcoming Temporal API (or a polyfill):
const legacyDate = new Date()
const timeZoneId = 'Asia/Tokyo'
const zonedDateTime = legacyDate.toTemporalInstant().toZonedDateTimeISO(timeZoneId)
const rfc9557 = zonedDateTime.toString()
// "2025-03-23T04:02:09.087+09:00[Asia/Tokyo]"