Describe the bug
dayjs.tz(instance, timezone) returns different values depending on the type of input. If the instance is of type string, then the value returned is correct and as per the documentation: it sets the time in the specified timezone. But if it is anything else, it doesn't work properly: it sets the time in the local timezone, then updates the timezone to the specified timezone.

For example, if this is the input:

  // local timezone: UTC + 5:30

  const tz = 'Asia/Tokyo'; // UTC + 9
  const date = new Date(2023, 9, 8, 0, 0, 0, 0);

  const a = dayjs.tz('2023-10-08', tz);
  const b = dayjs.tz(date, tz);
  const c = dayjs.tz(date.valueOf(), tz);
  const d = dayjs.tz({year: 2023, month: 9, day: 8}, tz);

  console.info('test a', a.toISOString(), a.format());
  console.info('test b', b.toISOString(), b.format());
  console.info('test c', c.toISOString(), c.format());
  console.info('test d', d.toISOString(), d.format());

then this is the output:

test a 2023-10-07T15:00:00.000Z 2023-10-08T00:00:00+09:00
test b 2023-10-07T18:30:00.000Z 2023-10-08T03:30:00+09:00
test c 2023-10-07T18:30:00.000Z 2023-10-08T03:30:00+09:00
test d 2023-10-07T18:30:00.000Z 2023-10-08T03:30:00+09:00

Expected behavior
The output should be the same for all types of instances.

Information

• Day.js Version: v1.11.11
• OS: Linux
• Browser: Firefox 126.0.1
• Time zone: UTC + 5:30