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src/lib/calendar.ts

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+export const BERLIN_TIME_ZONE = 'Europe/Berlin';
+
+export type CalendarDay = {
+	date: string;
+	dayNumber: number;
+	inSelectedMonth: boolean;
+	isToday: boolean;
+};
+
+const berlinDateFormatter = new Intl.DateTimeFormat('en-CA', {
+	timeZone: BERLIN_TIME_ZONE,
+	year: 'numeric',
+	month: '2-digit',
+	day: '2-digit'
+});
+
+function padded(value: number) {
+	return String(value).padStart(2, '0');
+}
+
+function dateParts(value: string | Date) {
+	const parts = berlinDateFormatter.formatToParts(
+		typeof value === 'string' ? new Date(value) : value
+	);
+	const read = (type: Intl.DateTimeFormatPartTypes) =>
+		Number(parts.find((part) => part.type === type)?.value);
+	return { year: read('year'), month: read('month'), day: read('day') };
+}
+
+export function berlinDateKey(value: string | Date) {
+	const parts = dateParts(value);
+	return `${parts.year}-${padded(parts.month)}-${padded(parts.day)}`;
+}
+
+export function berlinMonthKey(value: string | Date) {
+	return berlinDateKey(value).slice(0, 7);
+}
+
+export function normalizeMonthKey(value: string | null | undefined, fallback: string) {
+	if (!value || !/^\d{4}-(0[1-9]|1[0-2])$/.test(value)) return fallback;
+	return value;
+}
+
+export function shiftMonth(monthKey: string, delta: number) {
+	const [year, month] = monthKey.split('-').map(Number);
+	const shifted = new Date(Date.UTC(year, month - 1 + delta, 1));
+	return `${shifted.getUTCFullYear()}-${padded(shifted.getUTCMonth() + 1)}`;
+}
+
+export function formatMonthLabel(monthKey: string) {
+	const [year, month] = monthKey.split('-').map(Number);
+	return new Intl.DateTimeFormat('de-DE', {
+		timeZone: 'UTC',
+		month: 'long',
+		year: 'numeric'
+	}).format(new Date(Date.UTC(year, month - 1, 1)));
+}
+
+export function buildCalendarDays(monthKey: string, todayKey: string): CalendarDay[] {
+	const [year, month] = monthKey.split('-').map(Number);
+	const first = new Date(Date.UTC(year, month - 1, 1));
+	const mondayOffset = (first.getUTCDay() + 6) % 7;
+	const start = new Date(Date.UTC(year, month - 1, 1 - mondayOffset));
+	return Array.from({ length: 42 }, (_, index) => {
+		const date = new Date(start);
+		date.setUTCDate(start.getUTCDate() + index);
+		const dateKey = `${date.getUTCFullYear()}-${padded(date.getUTCMonth() + 1)}-${padded(date.getUTCDate())}`;
+		return {
+			date: dateKey,
+			dayNumber: date.getUTCDate(),
+			inSelectedMonth: dateKey.startsWith(monthKey),
+			isToday: dateKey === todayKey
+		};
+	});
+}