[Bug] roundTime_locale test fails in DST-observing timezones

[davesecops]

Version

6.0.0 (bug present on current master)

Link to Minimal Reproduction

N/A — this is a test-only bug. Run npm run test in any DST-observing timezone (US, EU, Australia, NZ, Brazil, etc.).

Steps to Reproduce

1. Clone apache/echarts, npm install
2. Set system timezone to any DST-observing zone (e.g., America/New_York, Europe/Berlin, Australia/Sydney)
3. Run npm run test
4. Observe roundTime_locale test failure in test/ut/spec/util/time.test.ts

Current Behavior

FAIL test/ut/spec/util/time.test.ts
  ● util/time › roundTime › roundTime_locale

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

    Expected: 528526800000
    Received: 528523200000

      at Object.<anonymous> (spec/util/time.test.ts:166:18)

The 3,600,000ms difference (exactly 1 hour) is the DST offset.

Expected Behavior

All unit tests should pass regardless of the machine's timezone.

Root Cause

The getISOTimezone() helper in time.test.ts computes the UTC offset from new Date(0) (January 1, 1970) and applies that fixed offset string to construct expected dates across different months:

function getISOTimezone(): string {
    const offsetMinutes = (new Date(0)).getTimezoneOffset(); // ← always January offset
    // ...
}

// Then used for October dates:
new Date(\`1986-10-01T00:00:00.000\${timezoneStr}\`)  // ← January offset applied to October

In DST-observing timezones, the offset in January differs from October. For example, US Eastern:

• January: UTC-5 (EST)
• October: UTC-4 (EDT)

So getISOTimezone() returns -05:00, but new Date('1986-10-01T00:00:00.000-05:00') constructs midnight EST, which is 1AM EDT. Meanwhile roundTime() correctly rounds to midnight local time (EDT), producing a 1-hour mismatch.

The roundTime_UTC test passes because it uses .toISOString() which is timezone-independent.

Environment

• OS: Any (macOS, Linux, Windows)
• Node: 18+
• Timezone: Any DST-observing timezone (fails in ~70% of world timezones)