/**
 * Checks whether a cached item has expired based on its TTL and creation time.
 *
 * @param ttlInSeconds - The TTL value in seconds. If null, the item never expires
 * @param createdAt - The creation timestamp in ISO format
 * @returns `true` if the item has expired, `false` otherwise
 *
 * @example
 * ```ts
 * // Item created 2 hours ago with 1 hour TTL - expired
 * const twoHoursAgo = new Date(Date.now() - 2 * 60 * 60 * 1000).toISOString();
 * isTtlExpired(3600, twoHoursAgo) // Returns true
 *
 * // Item created 30 minutes ago with 1 hour TTL - not expired
 * const thirtyMinutesAgo = new Date(Date.now() - 30 * 60 * 1000).toISOString();
 * isTtlExpired(3600, thirtyMinutesAgo) // Returns false
 *
 * // Item with no TTL never expires
 * isTtlExpired(null, twoHoursAgo) // Returns false
 * ```
 */
export declare const isTtlExpired: (ttlInSeconds: number | null, createdAt: string) => boolean;
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