/**
 * Exactly what `toISOString()` produces: four-digit year, `T` separator,
 * millisecond precision, literal `Z`. No offsets, no omitted milliseconds, no
 * expanded-year form — each of those breaks the lexicographic-order property.
 */
export declare const CANONICAL_TIMESTAMP_PATTERN: RegExp;
/**
 * True when `value` is a canonical ISO-8601 UTC instant.
 *
 * Two checks, in order: the pattern pins the *shape*, then a round trip
 * through `Date` pins *calendar validity* — `2026-02-30T00:00:00.000Z` matches
 * the pattern and is not an instant.
 */
export declare function isCanonicalTimestamp(value: unknown): value is string;
/**
 * Fail-closed gate for a caller-supplied instant.
 *
 * @param value The timestamp as received from the caller.
 * @param field Field name, so the failure names the contract that was missed
 *   rather than just the bad string.
 * @returns The same string, so call sites can validate and assign in one step.
 * @throws {ByokCoreError} code `timestamp_not_canonical`.
 */
export declare function assertCanonicalTimestamp(value: string, field: string): string;