Timezone plugin unexpected daylight calculation

Describe the bug When using the timezone plugin the date calculated by dayjs is unexpected for daylight datetime calculation, I’m comparing towards moment-timezone.

Expected behaviour

const moment = require('moment-timezone');
const dayjs = require('dayjs');
const utc = require('dayjs/plugin/utc');
const tz = require('dayjs/plugin/timezone');

dayjs.extend(utc);
dayjs.extend(tz);

const timezone = 'America/Vancouver';
const currentDate = new Date('2020-03-08T12:00-07:00');
const nowMoment = moment(currentDate);
const nowDayJs = dayjs(currentDate);

// milliseconds for currentDate: 1583694000000
console.log(nowDayJs.valueOf() === nowMoment.valueOf()); // -> true

// daylight
// https://www.timetemperature.com/canada/daylight_saving_time_canada.shtml
const plainDate = currentDate.toISOString().slice(0, 10); // 2020-08-08
const dlMoment = moment.tz(plainDate, timezone).set({
  hour: 12,
  minute: 30
});
const dlDayJs = dayjs.tz(plainDate, timezone)
  .set('hour', 12)
  .set('minute', 30);
console.log(dlDayJs.format() === dlMoment.format()); // -> false (true expected)
// 2020-03-07T12:30:00-08:00 !== 2020-03-08T12:30:00-07:00

Information

• dayjs: 1.8.33
• moment-timezone: 0.5.31
• OS: Ubuntu 18.04.4 LTS
• Nodejs 12.18.0
• Time zone: GMT-07:00 DST (Pacific Daylight Time)

[nfantone]

Timezone support seems to be quite broken at the moment, unfortunately.

dayjs.tz("11-20-2020", "America/Los_Angeles").format()
// '2020-11-20T00:00:00-08:00'

dayjs.tz("11-20-2020", "America/Los_Angeles").toISOString()
// '2020-11-20T09:00:00.000Z' <--- 09:00:00 is incorrect here, should be 08:00:00

Compare to moment:

moment.tz('11-20-2020', 'MM-DD-YYYY', 'America/Los_Angeles').toISOString()
// '2020-11-20T08:00:00.000Z'

[notable-nathan]

I have run in to the same issue. Here is my test case, which is a bit simpler than the one above:

Expected:

import dayjs from 'dayjs';
const utc = require('dayjs/plugin/utc');
const timezone = require('dayjs/plugin/timezone');
dayjs.extend(utc);
dayjs.extend(timezone);

function getDateInTimezone(date, TZ = 'America/New_York') {
  return dayjs.tz(date, TZ);
}

describe('dayjs.tz', function () {
  it('Handles DST', function () {
    expect(getDateInTimezone('2020-03-02').toDate()).toEqual(new Date('2020-03-02T05:00:00.000Z'));
    expect(getDateInTimezone('2020-03-09').toDate()).toEqual(new Date('2020-03-09T05:00:00.000Z'));
    // DST start
    expect(getDateInTimezone('2020-03-10').toDate()).toEqual(new Date('2020-03-10T04:00:00.000Z')); // !!! 
    expect(getDateInTimezone('2020-11-01').toDate()).toEqual(new Date('2020-11-01T04:00:00.000Z')); // !!!
    // DST end
    expect(getDateInTimezone('2020-11-02').toDate()).toEqual(new Date('2020-11-02T05:00:00.000Z'));
    expect(getDateInTimezone('2020-11-09').toDate()).toEqual(new Date('2020-11-09T05:00:00.000Z'));
  });
});

Actual:

  ● dayjs.tz › Handles DST

    expect(received).toEqual(expected) // deep equality

    Expected: 2020-03-10T04:00:00.000Z
    Received: 2020-03-09T04:00:00.000Z

And some information

dayjs: 1.8.35 OS: MacOS 10.15.5 Nodejs 14.10.1 Time zone: GMT-07:00 DST (Pacific Daylight Time)

---

Edit: I just noticed DST actually started in the early hours of March 8, so it appears dayjs may be incorrectly calculating when DST should start (March 9 should have already been using -4 offset, but instead dayjs.tz() didn’t start using -4 offset until March 10)