Summary

• Ensure parse_iso8601 returns None for invalid dates, times, and offsets.
• Reject invalid timezone components in ISO8601 offsets (for example +02:99).
• Use explicit None checks in generate_quarters and generate_years so falsy explicit values are preserved.
• Add regression tests for invalid ISO8601 values, start_quarter=0, and start_year=0.

Validation

• uv run pytest -q
• uv run ruff check .
• uv run ty check