/**
 * Return the exact length of a UTC interval in `unit`, as a real (possibly fractional) number.
 *
 * - Distinct from `intervalCountUtc`, which counts calendar `unit` boundaries *crossed* rather
 *   than measuring exact duration — an interval from 23:59 to 00:01 touches 2 day boundaries
 *   via `intervalCountUtc` but has an exact length of ~0.0014 days via `intervalLengthUtc`.
 * - Boundaries are UTC boundaries — no DST is involved.
 * - Returns `0` for a zero-length interval (`start === end`).
 * - Returns `null` on invalid input (unparseable start/end, `start > end`, unsupported unit,
 *   leap-second strings).
 *
 * @param start ISO UTC datetime string for the interval start
 * @param end ISO UTC datetime 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 intervalLengthUtc("2024-01-01T23:59:00Z", "2024-01-02T00:01:00Z", "day") // 0.001388888888888889
 * @example intervalLengthUtc("2024-01-01T23:59:00Z", "2024-01-02T00:01:00Z", "minute") // 2
 * @example intervalLengthUtc("2024-01-01T00:00:00Z", "2024-01-01T00:00:00Z", "day") // 0
 * @example intervalLengthUtc("invalid", "2024-01-02T00:00:00Z", "day") // null
 */
export declare function intervalLengthUtc(start: string, end: string, unit: string): number | null;