One formatter carries both the clock and the calendar day so the two can never disagree across a
zone boundary, and en-CA is what gives the date its YYYY-MM-DD shape.
Parameters
zone: string
An IANA zone name, such as Europe/London.
nowMs: number
The moment to read the zone at, as epoch milliseconds.
What a zone's own clock reads at a given moment.
One formatter carries both the clock and the calendar day so the two can never disagree across a zone boundary, and en-CA is what gives the date its YYYY-MM-DD shape.