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)