Getting wrong startOf('day') in a different timezone

[nikhilag]

Describe the bug
On calculating startOf('day') in a different timezone, I am getting the wrong result. Note that I tried this at 6 PM on November 12 in Asia/Kolkata timezone.
Code - dayjs.tz(new Date()).tz('America/New_York').startOf('day').toString()
Result - "Thu, 12 Nov 2020 15:30:00 GMT"

Expected behavior
It should be Thu, 12 Nov 2020 05:00:00 GMT

Information

• Day.js Version - Tried in browser console on day.js.org website
• OS - MacOS Mojave
• Browser - Chrome 86.0.4240.193
• Time zone: Asia/Kolkata

[nbudin]

I have a reproduction for this in the dayjs test suite, but I'm not sure how to fix it. Here is how I reproduced it in test/timezone.test.js:

it('startOf day across timezones', () => {
  const originalDay = dayjs('2010-01-01T00:00:00Z').tz('UTC')
  const startOfDay = originalDay.startOf('day')
  expect(startOfDay.valueOf()).toEqual(originalDay.valueOf())
})

(I also had to extend dayjs with the timezone plugin to make this run.)

I'm going to keep at this and see if I can come up with a fix, but if a maintainer has an idea I'd love to hear it since I'm not very familiar with the dayjs internals.

[nbudin]

I meant to paste the output from this test, sorry about that. Here it is:

$ env TZ=America/Whitehorse yarn run test-tz
yarn run v1.22.10
$ date && jest test/timezone.test --coverage=false
Sun Nov 15 11:43:34 MST 2020
 FAIL  test/timezone.test.js
  ✓ Add Time days (DST) (9ms)
  ✓ Utc Offset (1ms)
  ✓ Diff (DST) (5ms)
  ✓ UTC add day in DST (2ms)
  ✓ UTC and utcOffset (1ms)
  ✓ UTC diff in DST (1ms)
  ✕ startOf day across timezones (13ms)

  ● startOf day across timezones

    expect(received).toEqual(expected)

    Expected value to equal:
      1262304000000
    Received:
      1262275200000

      77 |   const originalDay = dayjs('2010-01-01T00:00:00Z').tz('UTC')
      78 |   const startOfDay = originalDay.startOf('day')
    > 79 |   expect(startOfDay.valueOf()).toEqual(originalDay.valueOf())
      80 | })
      81 |

      at Object.<anonymous> (test/timezone.test.js:79:32)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 6 passed, 7 total
Snapshots:   0 total
Time:        0.823s, estimated 1s
Ran all test suites matching /test\/timezone.test/i.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

[iamkun]

🎉 This issue has been resolved in version 1.9.7 🎉

The release is available on:

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

Your semantic-release bot 📦🚀