/**
 * Returns a string in the format 'HH:MM:SS.mmm' representing the duration between the given start and end times.
 * @param startTime The start time in milliseconds (shardusGetTime()).
 * @param endTime The end time in milliseconds. If not provided, the current time will be used.
 */
export declare function readableDuration(startTime: number, endTime?: number | undefined): string;