/**
 * Return how many distinct calendar dates two Unix epoch intervals share, in a given
 * time zone.
 *
 * - Counts the number of local dates touched by the closed intersection
 *   `[max(aStart, bStart), min(aEnd, bEnd)]` — inclusive of both endpoints.
 * - Uses the system timeZone by default (consistent with `addUnix` and
 *   `intervalCountUnix`), so day counts are host-dependent unless `timeZone` is given.
 * - 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 (non-finite/non-integer epoch values, invalid timeZone).
 * - Diverges from date-fns's `getOverlappingDaysInIntervals`, which rounds up elapsed
 *   24-hour periods instead of counting calendar dates. To reproduce date-fns's number,
 *   compose `intervalIntersectionUnix` with `intervalCountUnix`:
 *   `const span = intervalIntersectionUnix(aStart, aEnd, bStart, bEnd); span ? intervalCountUnix(span.start, span.end, "day") : 0;`
 *
 * @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
 * @param options optional: epochUnit ("seconds" | "milliseconds"), timeZone (IANA, default: system timeZone)
 * @returns number of shared calendar dates, `0` when disjoint, or null on invalid input
 *
 * @example intervalOverlappingDaysUnix(0, 172800000, 86400000, 259200000, { timeZone: "UTC" }) // 2
 * @example intervalOverlappingDaysUnix(0, 86400000, 86400000, 172800000, { timeZone: "UTC" }) // 1 (adjacent)
 * @example intervalOverlappingDaysUnix(0, 86400000, 172800000, 259200000, { timeZone: "UTC" }) // 0 (disjoint)
 * @example intervalOverlappingDaysUnix(NaN, 172800000, 86400000, 259200000, { timeZone: "UTC" }) // null
 */
export declare function intervalOverlappingDaysUnix(aStart: number | string, aEnd: number | string, bStart: number | string, bEnd: number | string, options?: {
    epochUnit?: "seconds" | "milliseconds";
    timeZone?: string;
}): number | null;