Warn for non-standard datetime values

[ryux1]

Summary

• validate non-empty datetime attributes against the ECMAScript date-time string format
• warn for malformed or implementation-dependent values while preserving the existing parsing behavior
• cover valid date-only, date-time, offset, expanded-year, and midnight forms

Closes #266

Testing

• npm test (636 Chromium tests)
• TypeScript and ESLint checks run as part of the test build
• npx prettier --check src/relative-time-element.ts test/relative-time.js
• git diff --check

[Copilot]

Copilot review overview

🟡 Changes recommended

Validation remains partly implementation-dependent, and the new test assumes V8-specific parsing behavior.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 2

Open (2)

• Require complete date before accepting a time suffix · New
• Avoid hard-coding engine-specific Date.parse results · New

What changed in this PR

Adds warnings for non-standard datetime values while preserving native parsing behavior.

Changes:

• Adds ECMAScript date-time validation and warnings.
• Adds coverage for valid and malformed values.

File	Description
src/​relative-time-element.ts	Adds validation and warning logic.
test/​relative-time.js	Tests warnings and accepted formats.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Copilot]

Copilot review overview

🔵 Needs a closer look

The regex accepts incomplete date-time forms such as 2024T03:04, preventing the intended warning.

Review effort: Balanced
Findings: 1

Open (1)

• Require complete date before accepting a time suffix

Resolved since last review (1)

• Avoid hard-coding engine-specific Date.parse results

Previously missed (1)

In code that hasn't changed since last review

Require complete dates before accepting a time component

src/​relative-time-element.ts:23

The time portion is optional independently of the month/day portion, so non-standard values such as 2024T03:04 and 2024-01T03:04 match this regex. V8's Date.parse also accepts both, which means Chromium emits no warning for exactly the implementation-dependent fallback this validation is intended to detect. Nest the T... group under the day component (and add these values to the malformed cases) so a time is only allowed after a complete date.

[francinelucca]

Thanks for this contribution @ryux1 🙏🏽, can you take a look at copilot's review comments above? thanks!