/**
 * Return how many distinct UTC calendar dates two UTC intervals share.
 *
 * - Counts the number of UTC dates touched by the closed intersection
 *   `[max(aStart, bStart), min(aEnd, bEnd)]` — inclusive of both endpoints.
 * - Boundaries are UTC boundaries — no DST is involved.
 * - Adjacent intervals (e.g. `aEnd === bStart`) share one date and count as `1`.
 * - Returns `0` when the intervals do not overlap at all (a well-defined answer, not
 *   invalid input).
 * - Returns `null` if either interval is invalid (`start > end`).
 * - Returns `null` on invalid input (wrong type, malformed strings, leap seconds).
 * - Diverges from date-fns's `getOverlappingDaysInIntervals`, which rounds up elapsed
 *   24-hour periods instead of counting calendar dates (its own doc example — Jan 10-20 vs
 *   Jan 17-21 — returns 3 there, 4 here). To reproduce date-fns's number, compose
 *   `intervalIntersectionUtc` with `intervalCountUtc`:
 *   `const span = intervalIntersectionUtc(aStart, aEnd, bStart, bEnd); span ? intervalCountUtc(span.start, span.end, "day") : 0;`
 *
 * @param aStart ISO 8601 UTC datetime string for the first interval start
 * @param aEnd ISO 8601 UTC datetime string for the first interval end
 * @param bStart ISO 8601 UTC datetime string for the second interval start
 * @param bEnd ISO 8601 UTC datetime string for the second interval end
 * @returns number of shared calendar dates, `0` when disjoint, or null on invalid input
 *
 * @example intervalOverlappingDaysUtc("2024-01-01T23:59:00Z", "2024-01-02T00:01:00Z", "2024-01-01T23:59:00Z", "2024-01-02T00:01:00Z") // 2
 * @example intervalOverlappingDaysUtc("2024-01-01T00:00:00Z", "2024-01-02T00:00:00Z", "2024-01-02T00:00:00Z", "2024-01-03T00:00:00Z") // 1 (adjacent)
 * @example intervalOverlappingDaysUtc("2024-01-01T00:00:00Z", "2024-01-02T00:00:00Z", "2024-01-03T00:00:00Z", "2024-01-04T00:00:00Z") // 0 (disjoint)
 * @example intervalOverlappingDaysUtc("invalid", "2024-06-30T23:59:59Z", "2024-04-01T00:00:00Z", "2024-12-31T23:59:59Z") // null
 */
export declare function intervalOverlappingDaysUtc(aStart: string, aEnd: string, bStart: string, bEnd: string): number | null;