/**
 * Return the start of the quarter for a given UTC datetime string.
 *
 * - Calculates quarter start month: Q1=1, Q2=4, Q3=7, Q4=10.
 * - Returns "" for invalid input.
 *
 * @param value ISO UTC datetime string (e.g. "2024-03-15T12:00:00Z")
 * @returns ISO UTC datetime string representing the start of the quarter, or "" on invalid input
 *
 * @example startOfQuarterForUtc("2024-03-15T12:00:00Z") // "2024-01-01T00:00:00Z"
 * @example startOfQuarterForUtc("invalid") // ""
 */
export declare function startOfQuarterForUtc(value: string): string;