/**
 * Converts an ISO date string (YYYY-MM-DD) to German display format (DD.MM.YYYY).
 * Returns an empty string for invalid or empty input.
 */
export function isoToGerman(iso: string): string {
	if (!iso || !/^\d{4}-\d{2}-\d{2}$/.test(iso)) return '';
	const [y, m, d] = iso.split('-');
	return `${d}.${m}.${y}`;
}

/**
 * Converts a German date string (DD.MM.YYYY) to ISO format (YYYY-MM-DD).
 * Returns an empty string for invalid or empty input.
 */
export function germanToIso(german: string): string {
	const match = german.match(/^(\d{2})\.(\d{2})\.(\d{4})$/);
	if (!match) return '';
	const [, d, m, y] = match;
	return `${y}-${m}-${d}`;
}