[I] [Bug] roundTime_locale test fails in DST-observing timezones [echarts]

["davesecops (via GitHub)" <gi...@apache.org>]

davesecops opened a new issue, #21538:
URL: https://github.com/apache/echarts/issues/21538

   ### 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:
   
   ```typescript
   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)


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