Understand date-fns-tz

date-fns-tz-test.ts

const utcDate = new Date("2022-06-15T05:00:00Z");
console.log("utcDate", utcDate, utcDate.toString());

const utcDate2 = set(utcDate, { hours: 1 }); // Always set in curren time
console.log("utcDate2", utcDate2, utcDate2.toString());

const zonedDate = utcToZonedTime(utcDate, "Asia/Jakarta"); // 5 + 7 = 12 in current timezone (pretend to be in local)
console.log("zoneDate", zonedDate, zonedDate.toString());

const zonedDate2 = set(zonedDate, { hours: 1 });
console.log("zoneDate2", zonedDate2, zonedDate2.toString()); // 1 in current timezone (pretend to be in local)

const backtoUtc = zonedTimeToUtc(zonedDate2, "Asia/Jakarta");
console.log("backtoUtc", backtoUtc, backtoUtc.toString()); // 25 - 7 = 18 in UTC or 20 in Europe/Paris

console.log(
  formatInTimeZone(backtoUtc, "Asia/Jakarta", "yyyy-MM-dd HH:mm:ss zzzz")
);

console.log(
  format(zonedDate2, "yyyy-MM-dd HH:mm:ss zzzz", {
    timeZone: "Asia/Jakarta",
  })
);
});

gistfile1.txt

  console.log
    utcDate 2022-06-15T05:00:00.000Z Wed Jun 15 2022 07:00:00 GMT+0200 (Central European Summer Time)

      at Object.<anonymous> (time.test.ts:118:13)

  console.log
    utcDate2 2022-06-14T23:00:00.000Z Wed Jun 15 2022 01:00:00 GMT+0200 (Central European Summer Time)

      at Object.<anonymous> (time.test.ts:121:13)

  console.log
    zoneDate 2022-06-15T10:00:00.000Z Wed Jun 15 2022 12:00:00 GMT+0200 (Central European Summer Time)

      at Object.<anonymous> (time.test.ts:124:13)

  console.log
    zoneDate2 2022-06-14T23:00:00.000Z Wed Jun 15 2022 01:00:00 GMT+0200 (Central European Summer Time)

      at Object.<anonymous> (time.test.ts:127:13)

  console.log
    backtoUtc 2022-06-14T18:00:00.000Z Tue Jun 14 2022 20:00:00 GMT+0200 (Central European Summer Time)

      at Object.<anonymous> (time.test.ts:130:13)

  console.log
    2022-06-15 01:00:00 Western Indonesia Time

      at Object.<anonymous> (time.test.ts:132:13)

  console.log
    2022-06-15 01:00:00 Western Indonesia Time

      at Object.<anonymous> (time.test.ts:135:13)