Fix calendar-aligned elapsed durations

[francinelucca]

Summary

• calculate whole month and year durations from calendar boundaries instead of fixed 30-day months
• remove the resulting stray days from exact month and year anniversaries
• include the regression cases from add test cases for superfluous days #263 and correct existing expectations that encoded the same bug

Fixes #262.

Testing

• npm run build
• full browser test suite (642 tests)

[Copilot]

Pull request overview

Updates elapsed durations to use calendar-aligned month and year anniversaries.

Changes:

• Adds calendar-based duration handling.
• Corrects affected expectations and adds regression cases.

Show a summary per file

File	Description
src/duration.ts	Adds calendar-aligned elapsed-time logic.
test/duration.ts	Updates month-duration expectations.
test/relative-time.js	Adds month/year regression coverage.

Review details

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Suppressed comments (1)

src/duration.ts:167

• Skipping the time comparison for every span of at least 12 months silently drops lower units even at second or millisecond precision. The added 2022-01-01 10:00 to 2023-01-01 09:00 case is only 364 days and 23 hours, yet this returns exactly -P1Y. Year spans should also require the time to match at the requested precision, or retain the remainder relative to the calendar anchor.

  // Treat matching calendar days at least a year apart as anniversaries even
  // when their times differ, rather than leaking fixed-month remainder days.
  const isAnniversary = Math.abs(calendarMonths) >= 12
  if (!isAnniversary && !hasSameTimeAtPrecision(date, reference, precisionIndex)) return

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Balanced