/**
 * Return the combined span of two Unix epoch intervals, or null when they are disjoint.
 *
 * - Compares numeric Unix epoch values directly.
 * - 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 (non-numeric types, non-finite values).
 *
 * @param aStart Unix epoch value (seconds or milliseconds) — first interval start
 * @param aEnd Unix epoch value (seconds or milliseconds) — first interval end
 * @param bStart Unix epoch value (seconds or milliseconds) — second interval start
 * @param bEnd Unix epoch value (seconds or milliseconds) — second interval end
 * @returns `{ start, end }` with the merged span, or null on invalid input / disjoint intervals
 *
 * @example intervalUnionUnix(0, 1700000000, 1000000, 2000000) // { start: 0, end: 1700000000 }
 * @example intervalUnionUnix(0, 1000000, 1000000, 2000000) // { start: 0, end: 2000000 }
 * @example intervalUnionUnix(0, 1000000, 1000001, 2000000) // null
 * @example intervalUnionUnix(NaN, 1700000000, 1000000, 2000000) // null
 * @example intervalUnionUnix("0", "1700000000", "1000000", "2000000") // { start: 0, end: 1700000000 }
 */
export declare function intervalUnionUnix(aStart: number | string, aEnd: number | string, bStart: number | string, bEnd: number | string): {
    start: number;
    end: number;
} | null;