/**
 * Return the start of the quarter for a given ISO datetime.
 *
 * - Returns the first day of the quarter with time set to 00:00:00.
 * - Q1 returns "01-01T00:00:00", Q2 returns "04-01T00:00:00", etc.
 * - Validates input using isValidDateTime.
 *
 * @param value ISO PlainDateTime string
 * @returns ISO PlainDateTime string for the start of the quarter, or "" on invalid input
 *
 * @example startOfQuarterForDateTime("2024-03-15T12:00:00") // "2024-01-01T00:00:00"
 * @example startOfQuarterForDateTime("2024-06-15T12:00:00") // "2024-04-01T00:00:00"
 * @example startOfQuarterForDateTime("invalid") // ""
 */
export declare function startOfQuarterForDateTime(value: string): string;