Duration between dates
A duration is the same span expressed in whichever unit suits it. Under a week, days. Under a year, weeks or months. Beyond that, years and months. The calendar breakdown at the top always reads naturally; the totals underneath are for calculations.
How to describe a duration
Choosing the unit is a communication decision rather than an arithmetic one. "Nine hundred days" and "two years five months" describe the same span, and they land very differently; the first sounds like an endurance, the second like a plan. Reports and contracts generally want the calendar form; countdowns and progress bars want the smallest unit that still moves visibly.
Questions
The largest one that still conveys precision. Years and months for long spans, days for anything under a month.
Because small units produce large numbers. It is a presentation choice, not a factual one.
For something you occupy, yes. For a gap between events, no.
By adding whole months to the start until one more would overshoot, then counting the days left.
Yes, using real month and year lengths throughout.