/**
 * Return the exact length of a datetime interval in `unit`, as a real (possibly fractional) number.
 *
 * - Distinct from `intervalCountDateTime`, which counts calendar/clock `unit` boundaries
 *   *crossed* rather than measuring exact duration — an interval from 23:59 to 00:01 touches 2
 *   day boundaries via `intervalCountDateTime` but has an exact length of ~0.0014 days via
 *   `intervalLengthDateTime`.
 * - Uses `Temporal.Duration.prototype.total`, which resolves calendar units (month, year)
 *   against the interval's own start so a partial month is expressed as a true fraction rather
 *   than truncated.
 * - Returns `0` for a zero-length interval (`start === end`).
 * - Returns `null` on invalid input (unparseable start/end, `start > end`, unsupported unit).
 *
 * @param start ISO PlainDateTime string for the interval start
 * @param end ISO PlainDateTime string for the interval end
 * @param unit unit string — any `DateTimeUnit`
 * @returns exact length of the interval expressed in `unit`, or null on invalid input
 *
 * @example intervalLengthDateTime("2024-01-01T23:59:00", "2024-01-02T00:01:00", "day") // 0.001388888888888889
 * @example intervalLengthDateTime("2024-01-01T23:59:00", "2024-01-02T00:01:00", "minute") // 2
 * @example intervalLengthDateTime("2024-01-01T00:00:00", "2024-01-01T00:00:00", "day") // 0
 * @example intervalLengthDateTime("invalid", "2024-01-02T00:00:00", "day") // null
 */
export declare function intervalLengthDateTime(start: string, end: string, unit: string): number | null;