/**
 * Return true when two UTC intervals are exactly adjacent — one's end equals the other's
 * start with zero gap and zero overlap.
 *
 * - Uses `Temporal.Instant.compare` for comparison.
 * - Returns `true` when `aEnd + 1 nanosecond === bStart` or `bEnd + 1 nanosecond === aStart`.
 * - Returns `false` when intervals overlap, are disjoint with a gap, or are invalid.
 * - Returns `false` 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 true if intervals are exactly adjacent, or false on invalid input
 *
 * @example intervalAbutsUtc("2024-01-01T09:00:00Z", "2024-06-30T12:00:00Z", "2024-06-30T12:00:00.000000001Z", "2024-12-31T17:00:00Z") // true
 * @example intervalAbutsUtc("2024-06-30T12:00:00Z", "2024-12-31T17:00:00Z", "2024-01-01T09:00:00Z", "2024-06-30T12:00:00.000000001Z") // true
 * @example intervalAbutsUtc("2024-01-01T09:00:00Z", "2024-06-30T12:00:00Z", "2024-06-30T12:00:01Z", "2024-12-31T17:00:00Z") // false (gap)
 * @example intervalAbutsUtc("2024-01-01T09:00:00Z", "2024-06-30T13:00:00Z", "2024-06-30T12:00:00Z", "2024-12-31T17:00:00Z") // false (overlap)
 * @example intervalAbutsUtc("invalid", "2024-06-30T12:00:00Z", "2024-06-30T12:00:00Z", "2024-12-31T17:00:00Z") // false
 */
export declare function intervalAbutsUtc(aStart: string, aEnd: string, bStart: string, bEnd: string): boolean;