dayOfYear/localizedFormat of Plugin Tests Fail in Non-UTC Timezones Due to Missing Parsed Timezone Info

[JasonLiu-1214]

Bug Report: Timezone Handling in Day.js Plugins

Describe the bug
When the local computer timezone is set to US/Los_Angeles, most plugin test cases fail. After the latest fixes, almost all test cases pass except for two:

• test/plugin/localizedFormat.test.js
• test/plugin/dayOfYear.test.js

These two are hard to fix because Day.js does not store the timezone information when parsing a string. Therefore, functions like dayOfYear and format can only operate based on the current environment's timezone. Fixing this properly would require a core feature change in Day.js to store the original string’s timezone, which would be a significant modification.

Expected behavior
All plugin test cases should pass regardless of the local system timezone, correctly handling dayOfYear and format when the input string contains a timezone offset.

Information

• Day.js Version: ALL
• OS: macOS / Windows (any OS, but tested on US/LA timezone)
• Browser: N/A (Node.js environment)
• Time zone: US/Pacific (GMT-07:00 DST, Pacific Daylight Time)

Test cases demonstrating the issue

describe('Timezone conversion format', () => {
  it('converts between fixed-offset timezones correctly', () => {
    expect(dayjs.tz('2025-03-09 12:00', 'Etc/GMT-5').tz('Etc/GMT+5').format())
      .toBe('2025-03-09T02:00:00-05:00');
    expect(dayjs.tz('2025-03-09 12:30', 'Etc/GMT-5').tz('Etc/GMT+5').format())
      .toBe('2025-03-09T02:30:00-05:00');
  });

  it('converts from fixed-offset timezone to IANA timezone correctly', () => {
    expect(dayjs('2025-03-08T21:00:00.000Z')
      .tz('Etc/GMT-5')
      .tz('America/Argentina/Buenos_Aires')
      .format()).toBe('2025-03-08T18:00:00-03:00');
  });
});

<img width="2370" height="1846" alt="Image" src="https://github.com/user-attachments/assets/6d33e4f5-afaf-4422-916a-98dba169c767" />

[mathieumaf]

Hi @JasonLiu-1214,

I looked into this and the issue is actually in the tests, not in the plugin code. The dayOfYear and localizedFormat plugins work correctly — they operate on local time, which is the expected behavior (consistent with Moment.js).

The tests were implicitly assuming UTC as the local timezone:

• dayOfYear.test.js: Uses UTC strings (2015-01-01T00:00:00.000Z) but asserts results with toISOString(). In UTC-8, this date resolves to Dec 31 2014 locally, so dayOfYear(4) operates on 2014 instead of 2015.
• localizedFormat.test.js: Uses new Date(0) (epoch = Jan 1 1970 00:00 UTC). In UTC-8, that's Dec 31 1969, so format('YYYY') returns 1969 instead of 1970.

No core changes are needed to store the original timezone. The fix is to make the tests timezone-agnostic by using local date strings instead of UTC ones.

I submitted a PR with the fix: #3027