/**
 * Return how many distinct calendar dates two zoned intervals share, counted in the
 * first interval's zone.
 *
 * - Counts the number of local dates touched by the closed intersection
 *   `[max(aStart, bStart), min(aEnd, bEnd)]` — inclusive of both endpoints.
 * - `aEnd`, `bStart`, and `bEnd` are re-expressed in `aStart`'s time zone before comparing —
 *   Temporal refuses to compute day-granularity differences directly across two zones, since
 *   day length varies with DST/offset changes. As a result this function is NOT commutative:
 *   swapping the two intervals can change the answer when their zones differ.
 * - Zones whose calendar skips a date entirely (e.g. `Pacific/Apia`'s 2011 dateline change)
 *   still count the skipped date — this matches `intervalCountZoned`'s calendar-arithmetic
 *   rule, so the two functions never disagree about what a day is.
 * - 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).
 * - **Accepts mixed calendar systems** (E7's D4-zoned, issue #152): both bare ISO zoned strings
 *   and GMT calendar-annotated ones (`"5784-06-15T14:30:00-05:00[u-ca=hebrew][America/New_York]"`),
 *   and the two endpoints need not agree on a calendar. Ordering is calendar-independent —
 *   verified that `Temporal.Instant` carries no calendar field at all and that
 *   `Instant.compare`/`ZonedDateTime.compare` both return `0` for the same instant expressed in
 *   hebrew, islamic-civil, japanese and iso8601.
 * - Still rejects Temporal's own `[timeZone][u-ca=...]` RFC 9557 ordering, which reads GMT's
 *   calendar-native digits as ISO digits — see `regex/calendar-zoned-date-time.ts`.
 * - 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 `intervalIntersectionZoned` with `intervalCountZoned`:
 *   `const span = intervalIntersectionZoned(aStart, aEnd, bStart, bEnd); span ? intervalCountZoned(span.start, span.end, "day") : 0;`
 *
 * @param aStart ISO 8601 zoned datetime string for the first interval start
 * @param aEnd ISO 8601 zoned datetime string for the first interval end
 * @param bStart ISO 8601 zoned datetime string for the second interval start
 * @param bEnd ISO 8601 zoned datetime string for the second interval end
 * @returns number of shared calendar dates (counted in aStart's zone), `0` when disjoint, or null on invalid input
 *
 * @example intervalOverlappingDaysZoned("2024-03-09T12:00:00-05:00[America/New_York]", "2024-03-11T12:00:00-04:00[America/New_York]", "2024-03-09T12:00:00-05:00[America/New_York]", "2024-03-11T12:00:00-04:00[America/New_York]") // 3 (spring-forward, 47 real hours)
 * @example intervalOverlappingDaysZoned("2024-01-01T00:00:00+00:00[UTC]", "2024-01-02T00:00:00+00:00[UTC]", "2024-01-03T00:00:00+00:00[UTC]", "2024-01-04T00:00:00+00:00[UTC]") // 0 (disjoint)
 * @example intervalOverlappingDaysZoned("invalid", "2024-06-30T23:59:59+00:00[UTC]", "2024-04-01T00:00:00+00:00[UTC]", "2024-12-31T23:59:59+00:00[UTC]") // null
 */
export declare function intervalOverlappingDaysZoned(aStart: string, aEnd: string, bStart: string, bEnd: string): number | null;