[timezone] ISO string and date object are being parsed differently

[dzonatan]

Describe the bug

There is a difference in result between what date type you provide when parsing under specific timezone.
This does not seem to be intentional. Let the code speak for itself:

const dateIso = '2019-07-03T00:00:00.000Z'; // doesn't really matter, could be any ISO8601 date
const dateObj = new Date(dateIso);

// local time (under GMT+3)
console.log(dayjs(dateIso).format());
// >> 2019-07-03T03:00:00+03:00 ✅
console.log(dayjs(dateObj).format());
// >> 2019-07-03T03:00:00+03:00 ✅

// utc
console.log(dayjs.utc(dateIso).format());
// >> 2019-07-03T00:00:00Z ✅
console.log(dayjs.utc(dateObj).format());
// >> 2019-07-03T00:00:00Z ✅

// timezone
console.log(dayjs.tz(dateIso, 'Europe/Stockholm').format());
// >> 2019-07-03T00:00:00+02:00 ❌ (-2 hours off), why?
console.log(dayjs.tz(dateObj, 'Europe/Stockholm').format());
// >> 2019-07-03T02:00:00+02:00 ✅

FIY: moment.js handles this uniformly

console.log(moment.tz(dateIso, 'Europe/Stockholm').format());
// >> 2019-07-03T02:00:00+02:00 ✅
console.log(moment.tz(dateObj, 'Europe/Stockholm').format());
// >> 2019-07-03T02:00:00+02:00 ✅

Information

• Day.js v1.10.7