Adding days across daylight savings does not work as expected

[denchen]

Describe the bug
I'm seeing a weird issue with using add() along with utc() across a daylight savings boundary where it doesn't add a full day.

Expected behavior
Given the following code:

var dayjs = require('dayjs');
var utc = require('dayjs/plugin/utc');
dayjs.extend(utc);

let testDate = dayjs('2019-03-10')
  .utc()
  .startOf('day');
console.log('TESTDATE: ', testDate.format());
let testDate2 = testDate.add(1, 'day');
console.log('TESTDATE2:', testDate2.format());

I expect it to output the following:

TESTDATE:  2019-03-10T00:00:00Z
TESTDATE2: 2019-03-11T00:00:00Z

Instead I see this:

TESTDATE:  2019-03-10T00:00:00Z
TESTDATE2: 2019-03-10T23:00:00Z

Replication here: https://runkit.com/accelerate/runkit-dayjs-daylight-savings

NOTE: I tested the same code with MomentJS and do get the expected behavior.

Information

• Day.js: 1.8.13
• OS: MacOS 10.14.4
• Chrome: 74.0.3729.108

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iamkun has been rewarded.

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Submitted pull Requests

• #590 fix: Fix UTC plugin add day DST bug

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[iamkun]

Your timezone, please? I can't get this reproduced.

[denchen]

GMT-07:00 DST (Pacific Daylight Time)

[iamkun]

Should be a bug related to UTC plugin and core bundle, investigating.

[IssueHuntBot]

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[iamkun]

🎉 This issue has been resolved in version 1.8.14 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[issuehunt-oss]

@iamkun has rewarded $90.00 to @iamkun. See it on IssueHunt

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[bbecker-te]

I think that some variant of this bug still may be happening. I am using version 1.11.6 of dayjs.

I have a simple test case for what I think is a dayjs bug. I add 2 days to a day in GMT/UTC timezone and get something that is not on a day boundary. It probably has something to do with daylight savings time because this is when daylight savings time happens in US. However, why its it a 2 hour difference, and why does it happen in GMT/UTC time? This change did not happen when we used moment.js.

it('should be exactly two days later when adding two days', function () {
    const day1 = dayjs.tz({ year: 2018, month: 2, day: 10 }, 'GMT');
    const day2 = day1.add(2, 'day');
    expect(day2.valueOf()).toEqual(dayjs.tz({ year: 2018, month: 2, day: 12, hour: 2 }, 'GMT').valueOf());
});