[1] https://tc39.es/proposal-temporal/docs/cookbook.html
const firstInMonth = Temporal.Now.with({ day: 1 });
Temporal.Now.plainDateISO().with({ day: 1 })
The “ISO” in there identifies the calendar. Want to know what the first of the current month in the Hebrew calendar is?
Temporal.Now.plainDate("hebrew").with({ day: 1 })