/**
 * Return the combined span of two time intervals, or null when they are disjoint.
 *
 * - Uses `Temporal.PlainTime.compare` for comparison.
 * - 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).
 *
 * @param aStart ISO 8601 time string for the first interval start
 * @param aEnd ISO 8601 time string for the first interval end
 * @param bStart ISO 8601 time string for the second interval start
 * @param bEnd ISO 8601 time string for the second interval end
 * @returns `{ start, end }` with the merged span, or null on invalid input / disjoint intervals
 *
 * @example intervalUnionTime("09:00:00", "17:00:00", "12:00:00", "18:00:00") // { start: "09:00:00", end: "18:00:00" }
 * @example intervalUnionTime("09:00:00", "17:00:00", "17:00:00", "18:00:00") // { start: "09:00:00", end: "18:00:00" }
 * @example intervalUnionTime("09:00:00", "17:00:00", "18:00:00", "20:00:00") // null
 * @example intervalUnionTime("invalid", "17:00:00", "12:00:00", "18:00:00") // null
 */
export declare function intervalUnionTime(aStart: string, aEnd: string, bStart: string, bEnd: string): {
    start: string;
    end: string;
} | null;