/**
 * Return the combined span of two UTC intervals, or null when they are disjoint.
 *
 * - Uses `Temporal.Instant.compare` for comparison (same instant semantics).
 * - Overlapping intervals return their merged span.
 * - Adjacent intervals (e.g. `aEnd === bStart`) share one instant and ARE merged.
 * - Returns `null` if either interval is invalid (`start > end`).
 * - Returns `null` on invalid input (wrong type, malformed strings, leap seconds).
 *
 * @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 `{ start, end }` with the merged span, or null on invalid input / disjoint intervals
 *
 * @example intervalUnionUtc("2024-01-01T00:00:00Z", "2024-06-30T23:59:59Z", "2024-04-01T00:00:00Z", "2024-12-31T23:59:59Z") // { start: "2024-01-01T00:00:00Z", end: "2024-12-31T23:59:59Z" }
 * @example intervalUnionUtc("2024-01-01T00:00:00Z", "2024-06-30T23:59:59Z", "2024-06-30T23:59:59Z", "2024-12-31T23:59:59Z") // { start: "2024-01-01T00:00:00Z", end: "2024-12-31T23:59:59Z" }
 * @example intervalUnionUtc("2024-01-01T00:00:00Z", "2024-06-30T23:59:59Z", "2024-07-01T00:00:00Z", "2024-12-31T23:59:59Z") // null
 * @example intervalUnionUtc("invalid", "2024-06-30T23:59:59Z", "2024-04-01T00:00:00Z", "2024-12-31T23:59:59Z") // null
 */
export declare function intervalUnionUtc(aStart: string, aEnd: string, bStart: string, bEnd: string): {
    start: string;
    end: string;
} | null;