js-joda convert UTC zoned datetime into local datetime

Question

I'm struggling to convert between a parsed UTC datetime into a LocalDateTime (America/New_York), with the hours adjusted accordingly between the two timezones, using the js-joda library.

In the javascript native Date object, what's stored is in UTC and is adjusted for the local (system) timezone at the time of display (.toString(), etc). I understand js-joda works differently, so I am assuming it's my own misunderstanding of just how js-joda timezones work. Regardless, I can't seem to get any kind of DateTime to report/show that it's 8am in the morning in New York, given a UTC time of 12pm noon.

Below is the Jest test-code to show what's been attempted so far.

import {
    DateTimeParseException,
    LocalDateTime,
    ZonedDateTime, ZoneId,
} from '@js-joda/core';
import '@js-joda/timezone';

// (Jest tests)
test('converting a utc zoned-datetime into a local datetime', () => {

    expect(ZoneId.systemDefault().id().toString()).toMatch('America/New_York');

    // America/New_York is currently -04:00 from UTC (daylight savings on)

    const utcStr1 = "2022-09-27T12:00:00Z";
    const utcStr2 = "2022-09-27T12:00:00+00:00";

    const utcDT = ZonedDateTime.parse(utcStr1);

    // make sure I understand how parsing UTC forms work:
    expect(utcDT).toEqual(ZonedDateTime.parse(utcStr2));  // OK
    expect(utcDT.zone().id()).toEqual(ZoneId.UTC.id());   // OK

    // trying to parse a UTC datetime string using LocalDateTime fails, 
    // so that won't work...
    expect(() => LocalDateTime.parse("2022-09-27T12:00:00Z")).toThrow(DateTimeParseException);
    expect(() => LocalDateTime.parse("2022-09-27T12:00:00+00:00")).toThrow(DateTimeParseException);

    /*
        How do I convert this UTC datetime into a LocalDateTime, adjusting for UTC (hr:12) -> America/New_York (hr:8)?
        All of the below attempts fail, returning 12.
     */

    expect(utcDT.toOffsetDateTime().hour()).toEqual(8); // FAILS: .hour() returns 12 not 8
    expect(utcDT.toLocalDateTime().hour()).toEqual(8);  // FAILS: .hour() returns 12 not 8
    expect(utcDT.withZoneSameLocal(ZoneId.systemDefault()).hour()).toEqual(8);  // FAILS: .hour() returns 12 not 8

});

Answer 1

The function to use is withZoneSameInstant:

const localDateTime = utcDT.withZoneSameInstant(ZoneId.systemDefault());