/**
 * Construct a Duration object based on start and end dates. Used internally by
 * `<FormatDuration>` and `<FormatRelativeTime>`.
 * @param start Start date
 * @param end End date
 * @returns A tuple containing the largest appropriate
 * Intl.RelativeTimeFormatUnit and the duration in that unit
 *
 * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DurationFormat/format
 */
export default function getDuration(start: Date, end: Date): [Intl.RelativeTimeFormatUnit, number];