feat: local time (#7)

Co-authored-by: terraputix <terraputix@mailbox.org>
Reviewed-on: useweb/open-meteo-blue#7

src/lib/utils/date.ts

@@ -0,0 +1,61 @@
+import { formatInTimeZone, toZonedTime } from 'date-fns-tz';
+import { isSameDay as isSameDayDateFns } from 'date-fns';
+
+/**
+ * Formats a UTC Date into a string for a specific timezone using date-fns patterns.
+ * Patterns: 'HH:mm' for 24h time, 'EEE' for short weekday, etc.
+ */
+export function formatZoned(date: Date, timeZone: string, pattern: string): string {
+	return formatInTimeZone(date, timeZone, pattern);
+}
+
+/**
+ * Checks if two dates are the same day in a specific timezone.
+ * Important for comparing weather forecast days against a selected date.
+ */
+export function isSameDayInZone(date1: Date, date2: Date, timeZone: string): boolean {
+	const z1 = toZonedTime(date1, timeZone);
+	const z2 = toZonedTime(date2, timeZone);
+	return isSameDayDateFns(z1, z2);
+}
+
+/**
+ * Gets the numeric hour (0-23) for a date in a specific timezone.
+ */
+export function getZonedHour(date: Date, timeZone: string): number {
+	return parseInt(formatInTimeZone(date, timeZone, 'H'), 10);
+}
+
+/**
+ * Returns a relative label like "Today", "Tomorrow", "Yesterday",
+ * or a formatted date string, all relative to the target timezone.
+ */
+export function getRelativeDayLabel(date: Date, timeZone: string): string {
+	const now = new Date();
+	const zonedDate = toZonedTime(date, timeZone);
+	const zonedNow = toZonedTime(now, timeZone);
+
+	if (isSameDayDateFns(zonedDate, zonedNow)) return 'Today';
+
+	const tomorrow = new Date(zonedNow);
+	tomorrow.setDate(tomorrow.getDate() + 1);
+	if (isSameDayDateFns(zonedDate, tomorrow)) return 'Tomorrow';
+
+	const yesterday = new Date(zonedNow);
+	yesterday.setDate(yesterday.getDate() - 1);
+	if (isSameDayDateFns(zonedDate, yesterday)) return 'Yesterday';
+
+	return formatInTimeZone(date, timeZone, 'EEE d MMM');
+}
+
+/**
+ * Formats a UTC offset in seconds to a string like "UTC+1" or "UTC-05:00"
+ */
+export function formatUtcOffset(offsetSeconds: number): string {
+	const sign = offsetSeconds >= 0 ? '+' : '-';
+	const abs = Math.abs(offsetSeconds);
+	const hours = Math.floor(abs / 3600);
+	const minutes = Math.floor((abs % 3600) / 60);
+	const pad = (n: number) => n.toString().padStart(2, '0');
+	return minutes === 0 ? `UTC${sign}${hours}` : `UTC${sign}${hours}:${pad(minutes)}`;
+}