Timezone conversion incorrect when daylight saving in effect

[PaulJNewell77]

Describe the bug
Dayjs does not seem to convert timezone correctly when DST is in effect. It produces different results from Moment. The Moment results seem intuitively what I'd expect, so believe to be correct unless there is some subtlety I'm missing.

Expected behavior
With this code:

    const when = '2020-07-30T12:00:00+00:00'; // Midday GMT in summer
    console.log('Moment London: ' + moment(when).tz('Europe/London').format('HH:mm Z'));
    console.log('Moment New York: ' + moment(when).tz('America/New_York').format('HH:mm Z'));

    console.log('Dayjs London: ' + dayjs(when).tz('Europe/London').format('HH:mm Z'));
    console.log('Dayjs New York: ' + dayjs(when).tz('America/New_York').format('HH:mm Z'));

The output is:

    Moment London: 13:00 +01:00
    Moment New York: 08:00 -04:00
    Dayjs London: 12:00 +00:00 // Expect 13:00 +01:00
    Dayjs New York: 08:00 -05:00 // Expect 08:00 -04:00

Information

• Day.js: v1.9.6
• OS: Windows 10 Home
• Nodejs: v12.8.3
• Time zone: GMT+00:00

[alexanderankin]

is this whats going wrong in the test suite of this library? i ran it and some tests were off by an hour.

[PaulJNewell77]

Hi, any thoughts on this issue @iamkun?

[lucasrcosta]

I can confirm that tests are failing in a locale affected by daylight savings (I'm in America/Vancouver).

I found out because tests stared breaking locally, and I made some experiments showing output different from Moment: https://runkit.com/lucasrcosta/5feca83aea7c09001b879b3a

I tried sorting it out on the code and removing the current offset from localUtcOffset on timezone/index.js fixed one test, but I couldn't figure out solutions for the others. It does seem related to not fixing DST.

Summary of all failing tests

 FAIL  test/plugin/dayOfYear.test.js
  ● DayOfYear set

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2015-01-04T00:00:00.000Z"
    Received:
      "2014-01-05T00:00:00.000Z"

      31 |   expect(dayjs('2015-01-01T00:00:00.000Z')
      32 |     .dayOfYear(4)
    > 33 |     .toISOString()).toBe('2015-01-04T00:00:00.000Z')
      34 | 
      35 |   expect(dayjs('2015-01-01T00:00:00.000Z')
      36 |     .dayOfYear(32)
      
      at Object.<anonymous> (test/plugin/dayOfYear.test.js:33:21)

 FAIL  test/plugin/localizedFormat.test.js
  ● Should not interpolate characters inside square brackets

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "1970 l 1970"
    Received:
      "1969 l 1969"

      29 | 
      30 |   expect(actualDate.format('[l]')).toBe('l')
    > 31 |   expect(actualDate.format('YYYY [l] YYYY')).toBe('1970 l 1970')
      32 |   expect(actualDate.format('l [l] l')).toBe('1/1/1970 l 1/1/1970')
      33 |   expect(actualDate.format('[L LL LLL LLLL]')).toBe(expectedDate.format('[L LL LLL LLLL]'))
      34 | 
      
      at Object.<anonymous> (test/plugin/localizedFormat.test.js:31:46)

 FAIL  test/plugin/timezone.test.js
  ● Parse › parse timestamp, js Date, Day.js object

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2020-08-07T12:00:00-07:00"
    Received:
      "2020-08-07T12:00:00-08:00"

      49 |     const Timestamp = dayjs.tz(d.getTime(), VAN)
      50 |     const Tmoment = moment.tz(d, VAN)
    > 51 |     expect(TjsDate.format()).toBe(result)
      52 |     expect(Tdayjs.format()).toBe(result)
      53 |     expect(Timestamp.format()).toBe(result)
      54 |     expect(Tmoment.format()).toBe(result)
      
      at Object.<anonymous> (test/plugin/timezone.test.js:51:30)

  ● Parse › parse and convert between timezones

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2014-06-01T09:00:00-07:00"
    Received:
      "2014-06-01T09:00:00-08:00"

      57 |   it('parse and convert between timezones', () => {
      58 |     const newYork = dayjs.tz('2014-06-01 12:00', NY)
    > 59 |     expect(newYork.tz('America/Los_Angeles').format()).toBe('2014-06-01T09:00:00-07:00')
      60 |     expect(newYork.tz('Europe/London').format()).toBe('2014-06-01T17:00:00+01:00')
      61 |   })
      62 | 
      
      at Object.<anonymous> (test/plugin/timezone.test.js:59:56)

  ● Convert › convert to target time

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2014-06-01T05:00:00-07:00"
    Received:
      "2014-06-01T05:00:00-08:00"

      74 |     const losAngeles = dayjs('2014-06-01T12:00:00Z').tz('America/Los_Angeles')
      75 |     const MlosAngeles = moment('2014-06-01T12:00:00Z').tz('America/Los_Angeles')
    > 76 |     expect(losAngeles.format()).toBe('2014-06-01T05:00:00-07:00')
      77 |     expect(losAngeles.format()).toBe(MlosAngeles.format())
      78 |     expect(losAngeles.valueOf()).toBe(1401624000000)
      79 |     expect(losAngeles.valueOf()).toBe(MlosAngeles.valueOf())
      
      at Object.<anonymous> (test/plugin/timezone.test.js:76:33)

  ● Convert › convert to target time

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2014-06-01T05:00:00-07:00"
    Received:
      "2014-06-01T05:00:00-08:00"

      85 |     [dayjs, moment].forEach((_) => {
      86 |       const losAngeles = _('2014-06-01T12:00:00Z').tz('America/Los_Angeles')
    > 87 |       expect(losAngeles.format()).toBe('2014-06-01T05:00:00-07:00')
      88 |       expect(losAngeles.valueOf()).toBe(1401624000000)
      89 |     })
      90 |   })
      
      at forEach (test/plugin/timezone.test.js:87:35)
          at Array.forEach (<anonymous>)
      at Object.<anonymous> (test/plugin/timezone.test.js:85:21)

  ● Convert › convert from time with timezone to target time

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2014-06-01T12:00:00Z"
    Received:
      "2014-06-01T12:00:00-01:00"

      93 |     const losAngelesInUTC = dayjs('2014-06-01T05:00:00-07:00').tz('UTC')
      94 |     const MlosAngelesInUTC = moment('2014-06-01T05:00:00-07:00').tz('UTC')
    > 95 |     expect(losAngelesInUTC.format()).toBe('2014-06-01T12:00:00Z')
      96 |     expect(losAngelesInUTC.format()).toBe(MlosAngelesInUTC.format())
      97 |   })
      98 | 
      
      at Object.<anonymous> (test/plugin/timezone.test.js:95:38)

  ● Convert › format Z

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "+09:00"
    Received:
      "+08:00"

      115 |     [dayjs, moment].forEach((_) => {
      116 |       const t = _('2020-08-06T03:48:10.258Z').tz(TOKYO)
    > 117 |       expect(t.format('Z')).toBe('+09:00')
      118 |     })
      119 |   })
      120 | })
      
      at forEach (test/plugin/timezone.test.js:117:29)
          at Array.forEach (<anonymous>)
      at Object.<anonymous> (test/plugin/timezone.test.js:115:21)

  ● DST, a time that never existed Spring Forward › 2012-03-11 02:00:00

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2012-03-11T03:00:00-04:00"
    Received:
      "2012-03-11T04:00:00-04:00"

      143 |     const d = dayjs.tz(s, NY)
      144 |     const m = moment.tz(s, NY)
    > 145 |     expect(d.format()).toBe('2012-03-11T03:00:00-04:00')
      146 |     expect(d.format()).toBe(m.format())
      147 |     expect(d.valueOf()).toBe(m.valueOf())
      148 |     expect(d.valueOf()).toBe(1331449200000)
      
      at Object.<anonymous> (test/plugin/timezone.test.js:145:24)

  ● DST, a time that never existed Spring Forward › 2012-03-11 02:59:59

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2012-03-11T03:59:59-04:00"
    Received:
      "2012-03-11T04:59:59-04:00"

      154 |     const d = dayjs.tz(s, NY)
      155 |     const m = moment.tz(s, NY)
    > 156 |     expect(d.format()).toBe('2012-03-11T03:59:59-04:00')
      157 |     expect(d.format()).toBe(m.format())
      158 |     expect(d.valueOf()).toBe(m.valueOf())
      159 |     expect(d.valueOf()).toBe(1331452799000)
      
      at Object.<anonymous> (test/plugin/timezone.test.js:156:24)

  ● DST, a time that never existed Spring Forward › 2012-03-11 03:00:00

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2012-03-11T03:00:00-04:00"
    Received:
      "2012-03-11T04:00:00-04:00"

      165 |     const d = dayjs.tz(s, NY)
      166 |     const m = moment.tz(s, NY)
    > 167 |     expect(d.format()).toBe('2012-03-11T03:00:00-04:00')
      168 |     expect(d.format()).toBe(m.format())
      169 |     expect(d.valueOf()).toBe(m.valueOf())
      170 |     expect(d.valueOf()).toBe(1331449200000)
      
      at Object.<anonymous> (test/plugin/timezone.test.js:167:24)

  ● DST, a time that never existed Fall Back › 2012-11-04 02:00:00

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2012-11-04T02:00:00-05:00"
    Received:
      "2012-11-04T01:00:00-05:00"

      204 |     [dayjs, moment].forEach((_) => {
      205 |       const d = _.tz(s, NY)
    > 206 |       expect(d.format()).toBe('2012-11-04T02:00:00-05:00')
      207 |       expect(d.utcOffset()).toBe(-300)
      208 |       expect(d.valueOf()).toBe(1352012400000)
      209 |     })
      
      at forEach (test/plugin/timezone.test.js:206:26)
          at Array.forEach (<anonymous>)
      at Object.<anonymous> (test/plugin/timezone.test.js:204:21)


Test Suites: 3 failed, 63 passed, 66 total
Tests:       12 failed, 601 passed, 613 total
Snapshots:   0 total
Time:        12.936s
Ran all test suites.

[addisonElliott]

This bug sounds similar to the recent comment in this one: #1340 (comment)

I'm going to cross-post the bug and how it can be fixed. Let me know if that doesn't actually fix this.

Not sure your bug is similar to the OPs, unless I'm missing the connection.

But I did confirm and figure out what the problem is with your case. It's actually a pretty easy fix (I think).

The problem

I'm in CST, which has an offset of -5/-6 (depending on DST). Let's use this as an example.

Given the input you gave: 2020-03-29T00:30:00Z

The code converts to local machines timezone (at this time it was -5 for me). Then it converts to the desired timezone. These are the results:

• 2020-03-28T20:30:00-05:00
• 2020-03-29T03:30:00+02:00

Then these two dates are subtracted (disregard the offset), giving -07:00. Next, we take local machine offset (-5) and subtract from -7. -5 - (-7) = +2. Correct answer.

That's how it should work. Let's look at the code and I'll show you the bug.

  proto.tz = function (timezone = defaultTimezone, keepLocalTime) {
    const oldOffset = this.utcOffset()
    const target = this.toDate().toLocaleString('en-US', { timeZone: timezone })
    const diff = Math.round((this.toDate() - new Date(target)) / 1000 / 60)
    let ins = d(target).$set(MS, this.$ms).utcOffset(localUtcOffset - diff, true)
    if (keepLocalTime) {
      const newOffset = ins.utcOffset()
      ins = ins.add(oldOffset - newOffset, MIN)
    }
    ins.$x.$timezone = timezone
    return ins
  }

Source: https://github.com/iamkun/dayjs/blob/dev/src/plugin/timezone/index.js#L101

Check out that it takes localUtcOffset and subtracts the diff. That localUtcOffset is obtained by doing the following. The problem is that DST is applied now so the offset for me is -06:00, giving the offset of +01:00 that we're seeing.

  const localUtcOffset = d().utcOffset()

Source: https://github.com/iamkun/dayjs/blob/dev/src/plugin/timezone/index.js#L39

The fix

Replace localUtcOffset with this.utcOffset() and that will return the UTC offset at that date

What you should do

I haven't tested this but I think it'll work.

If that works for you, I'd encourage you to submit a PR and fix this.

When was the bug introduced

Bug has existed ever since the timezone plugin was added.

https://github.com/iamkun/dayjs/pull/974/files

[addisonElliott]

A fix was applied in #1352 that may fix this bug. Can someone try with the latest changes and see if the issue still persists?

[lucasrcosta]

Still brakes here, although a little different...

Summary of all failing tests
 FAIL  test/plugin/timezone.test.js (5.167s)
  ● Convert › convert to target time

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      1401624000000
    Received:
      1401620400000

      76 |     expect(losAngeles.format()).toBe('2014-06-01T05:00:00-07:00')
      77 |     expect(losAngeles.format()).toBe(MlosAngeles.format())
    > 78 |     expect(losAngeles.valueOf()).toBe(1401624000000)
      79 |     expect(losAngeles.valueOf()).toBe(MlosAngeles.valueOf())
      80 |     expect(losAngeles.utcOffset()).toBe(-420)
      81 |     expect(losAngeles.utcOffset()).toBe(MlosAngeles.utcOffset())
      
      at Object.<anonymous> (test/plugin/timezone.test.js:78:34)

  ● Convert › convert to target time

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      1401624000000
    Received:
      1401620400000

      86 |       const losAngeles = _('2014-06-01T12:00:00Z').tz('America/Los_Angeles')
      87 |       expect(losAngeles.format()).toBe('2014-06-01T05:00:00-07:00')
    > 88 |       expect(losAngeles.valueOf()).toBe(1401624000000)
      89 |     })
      90 |   })
      91 | 
      
      at forEach (test/plugin/timezone.test.js:88:36)
          at Array.forEach (<anonymous>)
      at Object.<anonymous> (test/plugin/timezone.test.js:85:21)

  ● DST, a time that never existed Spring Forward › 2012-03-11 02:00:00

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2012-03-11T03:00:00-04:00"
    Received:
      "2012-03-11T04:00:00-04:00"

      145 |     const d = dayjs.tz(s, NY)
      146 |     const m = moment.tz(s, NY)
    > 147 |     expect(d.format()).toBe('2012-03-11T03:00:00-04:00')
      148 |     expect(d.format()).toBe(m.format())
      149 |     expect(d.valueOf()).toBe(m.valueOf())
      150 |     expect(d.valueOf()).toBe(1331449200000)
      
      at Object.<anonymous> (test/plugin/timezone.test.js:147:24)

  ● DST, a time that never existed Spring Forward › 2012-03-11 02:59:59

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2012-03-11T03:59:59-04:00"
    Received:
      "2012-03-11T04:59:59-04:00"

      156 |     const d = dayjs.tz(s, NY)
      157 |     const m = moment.tz(s, NY)
    > 158 |     expect(d.format()).toBe('2012-03-11T03:59:59-04:00')
      159 |     expect(d.format()).toBe(m.format())
      160 |     expect(d.valueOf()).toBe(m.valueOf())
      161 |     expect(d.valueOf()).toBe(1331452799000)
      
      at Object.<anonymous> (test/plugin/timezone.test.js:158:24)

  ● DST, a time that never existed Spring Forward › 2012-03-11 03:00:00

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2012-03-11T03:00:00-04:00"
    Received:
      "2012-03-11T04:00:00-04:00"

      167 |     const d = dayjs.tz(s, NY)
      168 |     const m = moment.tz(s, NY)
    > 169 |     expect(d.format()).toBe('2012-03-11T03:00:00-04:00')
      170 |     expect(d.format()).toBe(m.format())
      171 |     expect(d.valueOf()).toBe(m.valueOf())
      172 |     expect(d.valueOf()).toBe(1331449200000)
      
      at Object.<anonymous> (test/plugin/timezone.test.js:169:24)

  ● DST, a time that never existed Fall Back › 2012-11-04 02:00:00

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2012-11-04T02:00:00-05:00"
    Received:
      "2012-11-04T01:00:00-05:00"

      206 |     [dayjs, moment].forEach((_) => {
      207 |       const d = _.tz(s, NY)
    > 208 |       expect(d.format()).toBe('2012-11-04T02:00:00-05:00')
      209 |       expect(d.utcOffset()).toBe(-300)
      210 |       expect(d.valueOf()).toBe(1352012400000)
      211 |     })
      
      at forEach (test/plugin/timezone.test.js:208:26)
          at Array.forEach (<anonymous>)
      at Object.<anonymous> (test/plugin/timezone.test.js:206:21)

 FAIL  test/plugin/localizedFormat.test.js
  ● Should not interpolate characters inside square brackets

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "1970 l 1970"
    Received:
      "1969 l 1969"

      29 | 
      30 |   expect(actualDate.format('[l]')).toBe('l')
    > 31 |   expect(actualDate.format('YYYY [l] YYYY')).toBe('1970 l 1970')
      32 |   expect(actualDate.format('l [l] l')).toBe('1/1/1970 l 1/1/1970')
      33 |   expect(actualDate.format('[L LL LLL LLLL]')).toBe(expectedDate.format('[L LL LLL LLLL]'))
      34 | 
      
      at Object.<anonymous> (test/plugin/localizedFormat.test.js:31:46)

 FAIL  test/plugin/dayOfYear.test.js
  ● DayOfYear set

    expect(received).toBe(expected) // Object.is equality
    
    Expected value to be:
      "2015-01-04T00:00:00.000Z"
    Received:
      "2014-01-05T00:00:00.000Z"

      31 |   expect(dayjs('2015-01-01T00:00:00.000Z')
      32 |     .dayOfYear(4)
    > 33 |     .toISOString()).toBe('2015-01-04T00:00:00.000Z')
      34 | 
      35 |   expect(dayjs('2015-01-01T00:00:00.000Z')
      36 |     .dayOfYear(32)
      
      at Object.<anonymous> (test/plugin/dayOfYear.test.js:33:21)


Test Suites: 3 failed, 68 passed, 71 total
Tests:       8 failed, 648 passed, 656 total
Snapshots:   0 total
Time:        12.402s
Ran all test suites.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

dayjs on  dev is 📦 v0.0.0-development took 20s 982ms 
➜ git pull
Already up to date.```

[sts-ryan-holton]

I'm getting the same problem now that we're in BST, I'm in Europe/London timezone and this is what I'm getting:

• Date Time: 2021-04-15 14:00:00
• console.log(this.$dayjs.tz('2021-04-15 14:00:00', 'UTC').toISOString()) // gives 2021-04-15 14:00:00
• console.log(this.$dayjs.tz('2021-04-15 14:00:00', 'Europe/London').toISOString()) // gives 2021-04-15 13:00:00

Clearly wrong, any suggestions?

[hhamud]

Has this been fixed?. I am having the same issue with a few different time zones that apply daylight savings time, or has anyone been successful with any work around?,

[addisonElliott]

It sounds like it hasn't. I thought #1352 would fix it but others have reported not. Note that these changes are not officially released. In other words, you have to use the dev branch in order to get the latest changes.

If you're interested, try with the dev branch and see if it's still broken

[hhamud]

It sounds like it hasn't. I thought #1352 would fix it but others have reported not. Note that these changes are not officially released. In other words, you have to use the dev branch in order to get the latest changes.

If you're interested, try with the dev branch and see if it's still broken

Thanks for the tip. I've tried to install the dev branch however it still does not fix my issue.

[dobeerman]

Seems the issue still in place.

I tried to run this script in different time zones and got quite diverged results.

console.log(dayjs("2027-10-31T00:00:00Z").tz("Europe/Berlin").format());

console.log(dayjs("2027-10-31T01:00:00Z").tz("Europe/Berlin").format()); // +1hr

ServerTime: Wed Sep 15 2021 08:14:59 GMT+0200 (Central European Summer Time)

2027-10-31T02:00:00+02:00
2027-10-31T02:00:00Z

ServerTime: Wed Sep 15 2021 07:15:53 GMT+0100 (British Summer Time)

2027-10-31T02:00:00+03:00
2027-10-31T02:00:00+01:00

ServerTime: Wed Sep 15 2021 16:16:46 GMT+1000 (Vladivostok Standard Time)

2027-10-31T02:00:00+02:00
2027-10-31T02:00:00+01:00

This issue affects also development process when remote developers run it on their own local machines across the world.

I expect to have the same result which is not related to the server's timezone.

Please correct me if I'm wrong.

[CoreyKovalik]

Seems related to #1412

and also related to: #1408