Fix time conversion round-trip tests failing in CI

[alejandro-runner]

Problem

Round-trip conversion tests for unixAndTzidToIso8601() are failing in GitHub CI with a 24-hour (86400000ms) difference, but pass locally.

Failing Tests

• round-trips correctly with explicit UTC timezone
• handles edge cases around midnight with explicit UTC

Error

AssertionError: expected 86400000 to be less than 60000

Environment Differences

• Local: Tests pass (21/21)
• CI: Tests fail with 24-hour difference
• CI Environment: Ubuntu, Node.js 20, UTC timezone
• Local Environment: macOS, Node.js 22, America/Los_Angeles timezone

Investigation Needed

1. Why is there a 24-hour difference in CI but not locally?
2. Is Intl.DateTimeFormat with timeZoneName: 'longOffset' producing different results in CI?
3. Are date components being formatted differently in CI?
4. Is there a timezone data difference between environments?

Current Implementation

The function uses Intl.DateTimeFormat with timeZoneName: 'longOffset' to get the offset directly, which should be environment-independent, but something is still different in CI.

Related

Part of Issue #150 - Create Time Conversion Utilities