package.json

@@ -49,9 +49,9 @@
 		"typescript-eslint": "^8.48.1",
 		"vite": "^7.2.6",
 		"vitest": "^4.0.15",
-		"vitest-browser-svelte": "^2.0.1"
-	},
-	"dependencies": {
+		"vitest-browser-svelte": "^2.0.1",
+		"date-fns": "^4.1.0",
+		"date-fns-tz": "^3.2.0",
 		"echarts": "^6.0.0",
 		"mode-watcher": "^1.1.0",
 		"openmeteo": "^1.2.3"

src/lib/services/weather.ts

@@ -39,18 +39,21 @@ export function range(start: number, stop: number, step: number): number[] {
 /**
  * Extracts timestamp array (in milliseconds, with UTC offset applied) from a VariablesWithTime block.
  */
-export function getTimestamps(timeBlock: VariablesWithTime, utcOffsetSeconds: number): number[] {
+export function getTimestamps(timeBlock: VariablesWithTime): number[] {
 	const start = Number(timeBlock.time());
 	const end = Number(timeBlock.timeEnd());
 	const interval = timeBlock.interval();
-	return range(start, end, interval).map((t) => (t + utcOffsetSeconds) * 1000);
+	return range(start, end, interval).map((t) => t * 1000);
 }
 
 /**
  * Extracts Date array (with UTC offset applied) from a VariablesWithTime block.
  */
-export function getDates(timeBlock: VariablesWithTime, utcOffsetSeconds: number): Date[] {
-	return getTimestamps(timeBlock, utcOffsetSeconds).map((t) => new Date(t));
+export function getDates(timeBlock: VariablesWithTime): Date[] {
+	return getTimestamps(timeBlock).map((t) => {
+		const date = new Date(t);
+		return date;
+	});
 }
 
 /**
@@ -148,6 +151,7 @@ export function unitToDisplayString(unit: Unit): string {
 export interface WeatherLocation {
 	latitude: number;
 	longitude: number;
+	timezone?: string;
 }
 
 export interface WeatherUnitParams {
@@ -164,7 +168,6 @@ export interface WeekForecastParams extends WeatherLocation, WeatherUnitParams {
 	model?: string;
 	forecast_days?: number;
 	past_days?: number;
-	timezone?: string;
 }
 
 export interface WeekHourlyData {
@@ -197,6 +200,7 @@ export interface WeekForecastResult {
 	hourly: WeekHourlyData;
 	daily: WeekDailyData;
 	utcOffsetSeconds: number;
+	timezone: string;
 	hourlyTimestamps: number[];
 	hourlyDates: Date[];
 	dailyDates: Date[];
@@ -219,6 +223,7 @@ export interface ModelCompareResult {
 	models: ModelSeriesData[];
 	timestamps: number[];
 	utcOffsetSeconds: number;
+	timezone: string;
 	markAreas: MarkArea[];
 	units: Record<string, string>;
 	/** Flat record compatible with the existing chart utilities (keys like "temperature_2m_icon_seamless") */
@@ -246,6 +251,7 @@ export interface EnsembleForecastResult {
 	variables: Record<string, EnsembleVariableData>;
 	timestamps: number[];
 	utcOffsetSeconds: number;
+	timezone: string;
 	markAreas: MarkArea[];
 	/** Flat record compatible with existing chart utilities (keys like "temperature_2m_member00") */
 	hourlyFlat: Record<string, number[]>;
@@ -314,12 +320,13 @@ export async function fetchWeekForecast(params: WeekForecastParams): Promise<Wee
 	const responses = await fetchWeatherApi(FORECAST_URL, cleanParams);
 	const response = responses[0];
 	const utcOffsetSeconds = response.utcOffsetSeconds();
+	const timezone = response.timezone() ?? params.timezone ?? 'UTC';
 
 	const hourlyBlock = response.hourly()!;
 	const dailyBlock = response.daily()!;
 
 	// Hourly: variables are in the same order as WEEK_HOURLY_VARS
-	const hourlyTimestamps = getTimestamps(hourlyBlock, utcOffsetSeconds);
+	const hourlyTimestamps = getTimestamps(hourlyBlock);
 	const hourlyDates = hourlyTimestamps.map((t) => new Date(t));
 
 	const hourly: WeekHourlyData = {
@@ -336,7 +343,7 @@ export async function fetchWeekForecast(params: WeekForecastParams): Promise<Wee
 	};
 
 	// Daily: variables are in the same order as WEEK_DAILY_VARS
-	const dailyDates = getDates(dailyBlock, utcOffsetSeconds);
+	const dailyDates = getDates(dailyBlock);
 
 	const sunriseVar = dailyBlock.variables(3)!;
 	const sunsetVar = dailyBlock.variables(4)!;
@@ -354,12 +361,13 @@ export async function fetchWeekForecast(params: WeekForecastParams): Promise<Wee
 		winddirection_10m_dominant: getValues(dailyBlock.variables(9)!)
 	};
 
-	const markAreas = buildDaylightMarkAreas(daily.sunrise, daily.sunset, utcOffsetSeconds);
+	const markAreas = buildDaylightMarkAreas(daily.sunrise, daily.sunset);
 
 	return {
 		hourly,
 		daily,
 		utcOffsetSeconds,
+		timezone,
 		hourlyTimestamps,
 		hourlyDates,
 		dailyDates,
@@ -395,10 +403,10 @@ export async function fetchModelComparison(
 	// With multiple models, we get one response per model
 	const firstResponse = responses[0];
 	const utcOffsetSeconds = firstResponse.utcOffsetSeconds();
+	const timezone = firstResponse.timezone() ?? params.timezone ?? 'UTC';
 
-	// Extract timestamps from the first response's hourly block
 	const hourlyBlock = firstResponse.hourly()!;
-	const timestamps = getTimestamps(hourlyBlock, utcOffsetSeconds);
+	const timestamps = getTimestamps(hourlyBlock);
 
 	// Extract sunrise/sunset from the first response's daily block
 	let markAreas: MarkArea[] = [];
@@ -408,7 +416,7 @@ export async function fetchModelComparison(
 		const sunsetVar = dailyBlock.variables(1)!;
 		const sunrise = getInt64Values(sunriseVar);
 		const sunset = getInt64Values(sunsetVar);
-		markAreas = buildDaylightMarkAreas(sunrise, sunset, utcOffsetSeconds);
+		markAreas = buildDaylightMarkAreas(sunrise, sunset);
 	}
 
 	// Process each model's response
@@ -463,6 +471,7 @@ export async function fetchModelComparison(
 		models,
 		timestamps,
 		utcOffsetSeconds,
+		timezone,
 		markAreas,
 		units,
 		hourlyFlat,
@@ -511,9 +520,10 @@ export async function fetchEnsembleForecast(
 
 	const ensembleResponse = ensembleResponses[0];
 	const utcOffsetSeconds = ensembleResponse.utcOffsetSeconds();
+	const timezone = ensembleResponse.timezone() ?? params.timezone ?? 'UTC';
 
 	const hourlyBlock = ensembleResponse.hourly()!;
-	const timestamps = getTimestamps(hourlyBlock, utcOffsetSeconds);
+	const timestamps = getTimestamps(hourlyBlock);
 	const timeLength = timestamps.length;
 
 	// Extract sunrise/sunset for mark areas
@@ -524,7 +534,7 @@ export async function fetchEnsembleForecast(
 		if (dailyBlock) {
 			const sunrise = getInt64Values(dailyBlock.variables(0)!);
 			const sunset = getInt64Values(dailyBlock.variables(1)!);
-			markAreas = buildDaylightMarkAreas(sunrise, sunset, dailyResponse.utcOffsetSeconds());
+			markAreas = buildDaylightMarkAreas(sunrise, sunset);
 		}
 	}
 
@@ -613,6 +623,7 @@ export async function fetchEnsembleForecast(
 		variables,
 		timestamps,
 		utcOffsetSeconds,
+		timezone,
 		markAreas,
 		hourlyFlat,
 		hourlyUnitsFlat

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)}`;
+}

src/lib/utils/echarts/index.ts

@@ -54,7 +54,6 @@ export {
 	buildSpreadSeries,
 	calculateAverage,
 	calculateSpread,
-	convertTimestamps,
 	findUnit
 } from './series';
 export type {

src/lib/utils/echarts/options.ts

@@ -5,6 +5,7 @@
  * These builders ensure visual consistency across all chart pages and reduce
  * boilerplate in page-level components.
  */
+import { formatZoned } from '../date';
 import { getThemeColors } from './theme';
 
 import type { ThemeColors } from './theme';
@@ -30,6 +31,7 @@ export interface LegendOptions {
 
 export interface TooltipOptions {
 	unit: string;
+	timezone?: string;
 }
 
 export interface AxisOptions {
@@ -180,9 +182,9 @@ export function buildLegend(options: LegendOptions, colors?: ThemeColors): Recor
 // ─── X Axis (Time) ───────────────────────────────────────────────────────────
 
 /**
- * Builds a time-based X axis with theme-aware styling.
+ * Builds a time-based X axis with theme-aware styling and timezone-aware labels.
  */
-export function buildTimeXAxis(colors?: ThemeColors): Record<string, unknown> {
+export function buildTimeXAxis(timezone?: string, colors?: ThemeColors): Record<string, unknown> {
 	const c = colors ?? getThemeColors();
 
 	return {
@@ -197,7 +199,10 @@ export function buildTimeXAxis(colors?: ThemeColors): Record<string, unknown> {
 		},
 		axisLabel: {
 			color: c.text,
-			hideOverlap: true
+			hideOverlap: true,
+			formatter: timezone
+				? (value: number) => formatZoned(new Date(value), timezone, 'HH:mm')
+				: undefined
 		},
 		axisTick: {
 			lineStyle: {
@@ -336,6 +341,7 @@ export interface ChartOptionParams {
 	toolbox?: ToolboxOptions | false;
 	showCredit?: boolean;
 	colors?: ThemeColors;
+	timezone?: string;
 }
 
 /**
@@ -358,7 +364,7 @@ export function composeChartOption(params: ChartOptionParams): Record<string, un
 			hasSubtitle: hasTitle && !!params.title?.subtext,
 			showLegend
 		}),
-		xAxis: buildTimeXAxis(colors),
+		xAxis: buildTimeXAxis(params.timezone, colors),
 		yAxis: buildValueYAxis(params.yAxis, colors),
 		series: params.series,
 		textStyle: {

src/lib/utils/echarts/series.ts

@@ -5,9 +5,8 @@
  * weather chart visualizations. These builders encapsulate the styling
  * and configuration details so page-level code only needs to provide data.
  */
-
-import { CHART_COLORS } from './theme';
 import { isColumnUnit } from './options';
+import { CHART_COLORS } from './theme';
 
 // ─── Types ───────────────────────────────────────────────────────────────────
 
@@ -38,9 +37,7 @@ export interface CurrentTimeSeriesParams {
 
 export interface DaylightSeriesParams {
 	/** Array of mark area pairs: [[start, end], [start, end], ...] */
-	markAreas: Array<
-		[{ xAxis: number; itemStyle: { color: string } }, { xAxis: number }]
-	>;
+	markAreas: Array<[{ xAxis: number; itemStyle: { color: string } }, { xAxis: number }]>;
 }
 
 export interface SpreadSeriesParams {
@@ -125,9 +122,7 @@ export function buildAverageSeries(params: AverageSeriesParams): Record<string,
  * Builds a helper series that renders a vertical red line at the current time.
  * Uses an empty data series with a markLine to overlay onto the chart.
  */
-export function buildCurrentTimeSeries(params: CurrentTimeSeriesParams): Record<string, unknown> {
-	const { utcOffsetSeconds } = params;
-
+export function buildCurrentTimeSeries(): Record<string, unknown> {
 	return {
 		name: 'Current Time',
 		type: 'line',
@@ -137,7 +132,7 @@ export function buildCurrentTimeSeries(params: CurrentTimeSeriesParams): Record<
 			symbol: 'none',
 			data: [
 				{
-					xAxis: Date.now() + utcOffsetSeconds * 1000,
+					xAxis: Date.now(),
 					lineStyle: {
 						color: CHART_COLORS.currentTimeLine,
 						width: 2,
@@ -164,18 +159,17 @@ export function buildCurrentTimeSeries(params: CurrentTimeSeriesParams): Record<
  */
 export function buildDaylightMarkAreas(
 	sunrise: number[],
-	sunset: number[],
-	utcOffsetSeconds: number
+	sunset: number[]
 ): Array<[{ xAxis: number; itemStyle: { color: string } }, { xAxis: number }]> {
 	return sunrise.map((r: number, i: number) => [
 		{
-			xAxis: (r + utcOffsetSeconds) * 1000,
+			xAxis: r * 1000,
 			itemStyle: {
 				color: CHART_COLORS.daylight
 			}
 		},
 		{
-			xAxis: (sunset[i] + utcOffsetSeconds) * 1000
+			xAxis: sunset[i] * 1000
 		}
 	]);
 }
@@ -184,9 +178,7 @@ export function buildDaylightMarkAreas(
  * Builds a helper series that renders day/night shading bands via markArea.
  * Returns null if no mark areas are provided (so callers can filter it out).
  */
-export function buildDaylightSeries(
-	params: DaylightSeriesParams
-): Record<string, unknown> | null {
+export function buildDaylightSeries(params: DaylightSeriesParams): Record<string, unknown> | null {
 	if (params.markAreas.length === 0) return null;
 
 	return {
@@ -285,7 +277,7 @@ export function calculateAverage(
 		if (!model.startsWith(variable)) continue;
 
 		for (const [index, val] of (values as number[]).entries()) {
-			if (val !== null && val !== undefined) {
+			if (val !== null && val !== undefined && isFinite(val)) {
 				average[index] += val;
 				averageCount[index]++;
 			}
@@ -338,13 +330,6 @@ export function calculateSpread(
 	return { minValues, maxValues };
 }
 
-/**
- * Converts raw unix timestamps (seconds) to millisecond timestamps with UTC offset applied.
- */
-export function convertTimestamps(times: number[], utcOffsetSeconds: number): number[] {
-	return times.map((t) => (t + utcOffsetSeconds) * 1000);
-}
-
 /**
  * Finds the unit string for a given variable from the hourly_units map.
  * Returns an empty string if the variable is not found.

src/routes/weather/14-day/[location]/+page.svelte

@@ -55,7 +55,7 @@
 	interface FetchedData {
 		ensembleResult: EnsembleForecastResult;
 		timestamps: number[];
-		utc_offset_seconds: number;
+		timezone: string;
 		markAreas: MarkArea[];
 	}
 
@@ -102,15 +102,17 @@
 				forecast_days: 14,
 				temperature_unit: params.temperature_unit as 'celsius' | 'fahrenheit',
 				wind_speed_unit: params.wind_speed_unit as 'kmh' | 'ms' | 'mph' | 'kn',
-				precipitation_unit: params.precipitation_unit as 'mm' | 'inch'
+				precipitation_unit: params.precipitation_unit as 'mm' | 'inch',
+				timezone: loc.timezone
 			});
 
 			fetchedData = {
 				ensembleResult: result,
 				timestamps: result.timestamps,
-				utc_offset_seconds: result.utcOffsetSeconds,
+				timezone: result.timezone,
 				markAreas: result.markAreas
 			};
+			console.log(fetchedData.timezone);
 
 			loading = false;
 		};
@@ -123,7 +125,7 @@
 	$effect(() => {
 		if (!fetchedData) return;
 
-		const { ensembleResult, timestamps, utc_offset_seconds, markAreas } = fetchedData;
+		const { ensembleResult, timestamps, timezone, markAreas } = fetchedData;
 		const _showLegend = showLegend;
 
 		const colors = getThemeColors();
@@ -150,7 +152,7 @@
 			const averageData = average.map((val, idx) => [timestamps[idx], val] as [number, number]);
 			series.push(buildAverageSeries({ variable, data: averageData, unit }));
 
-			series.push(buildCurrentTimeSeries({ utcOffsetSeconds: utc_offset_seconds }));
+			series.push(buildCurrentTimeSeries());
 
 			const daylightSeries = buildDaylightSeries({ markAreas });
 			if (daylightSeries) {
@@ -167,7 +169,7 @@
 							subtext: `Compare ${params.hourly?.join(', ') || ''} in models: ${params.models?.join(', ') || ''}`
 						}
 					: null,
-				tooltip: { unit },
+				tooltip: { unit, timezone },
 				legend: {
 					show: _showLegend,
 					data: [variable + '_average']
@@ -181,7 +183,8 @@
 				series,
 				toolbox: false,
 				showCredit: isLast,
-				colors
+				colors,
+				timezone
 			});
 
 			newOptions.push(option);

src/routes/weather/compare/[location]/+page.svelte

@@ -68,7 +68,7 @@
 	interface FetchedData {
 		hourly: Record<string, unknown>;
 		hourly_units: Record<string, string>;
-		utc_offset_seconds: number;
+		timezone: string;
 		markAreas: MarkArea[];
 		timestamps: number[];
 	}
@@ -115,13 +115,14 @@
 				models: modelList,
 				temperature_unit: params.temperature_unit as 'celsius' | 'fahrenheit',
 				wind_speed_unit: params.wind_speed_unit as 'kmh' | 'ms' | 'mph' | 'kn',
-				precipitation_unit: params.precipitation_unit as 'mm' | 'inch'
+				precipitation_unit: params.precipitation_unit as 'mm' | 'inch',
+				timezone: loc.timezone
 			});
 
 			fetchedData = {
 				hourly: result.hourlyFlat,
 				hourly_units: result.hourlyUnitsFlat,
-				utc_offset_seconds: result.utcOffsetSeconds,
+				timezone: result.timezone,
 				markAreas: result.markAreas,
 				timestamps: result.timestamps
 			};
@@ -137,13 +138,7 @@
 	$effect(() => {
 		if (!fetchedData) return;
 
-		const {
-			hourly: hourlyData,
-			hourly_units,
-			utc_offset_seconds,
-			markAreas,
-			timestamps
-		} = fetchedData;
+		const { hourly: hourlyData, hourly_units, timezone, markAreas, timestamps } = fetchedData;
 		const _showLegend = showLegend;
 
 		const colors = getThemeColors();
@@ -178,7 +173,7 @@
 			const averageData = average.map((val, idx) => [timestamps[idx], val] as [number, number]);
 			series.push(buildAverageSeries({ variable, data: averageData, unit }));
 
-			series.push(buildCurrentTimeSeries({ utcOffsetSeconds: utc_offset_seconds }));
+			series.push(buildCurrentTimeSeries());
 
 			const daylightSeries = buildDaylightSeries({ markAreas });
 			if (daylightSeries) {
@@ -195,7 +190,7 @@
 							subtext: `Compare ${params.hourly?.join(', ') || ''} in models: ${params.models?.join(', ') || ''}`
 						}
 					: null,
-				tooltip: { unit },
+				tooltip: { unit, timezone },
 				legend: { show: _showLegend },
 				grid: {
 					hasTitle: isFirst,
@@ -206,7 +201,8 @@
 				series,
 				toolbox: false,
 				showCredit: isLast,
-				colors
+				colors,
+				timezone
 			});
 
 			newOptions.push(option);

src/routes/weather/week/[location]/+page.svelte

@@ -63,12 +63,14 @@
 				wind_speed_unit: params.wind_speed_unit as 'kmh' | 'ms' | 'mph' | 'kn',
 				precipitation_unit: params.precipitation_unit as 'mm' | 'inch',
 				forecast_days: 7,
-				past_days: 0
+				past_days: 0,
+				timezone: loc.timezone
 			});
 
 			fetchedHourly = {
 				hourly: result.hourly,
 				utc_offset_seconds: result.utcOffsetSeconds,
+				timezone: result.timezone,
 				timestamps: result.hourlyTimestamps,
 				hourlyDates: result.hourlyDates,
 				markAreas: result.markAreas
@@ -76,6 +78,7 @@
 
 			fetchedDaily = {
 				daily: result.daily,
+				timezone: result.timezone,
 				dailyDates: result.dailyDates
 			};
 

src/routes/weather/week/[location]/DailyCards.svelte

@@ -1,9 +1,11 @@
 <script lang="ts">
 	import { fade } from 'svelte/transition';
 
+	import { formatZoned, getRelativeDayLabel, isSameDayInZone } from '$lib/utils/date';
+
 	import { getTempStyle } from '../../utils/colors';
 	import weatherCodes from '../../utils/weather-codes';
-	import { type FetchedDaily, type WeatherUnits, getDayLabel, getWindArrowRotation } from './types';
+	import { type FetchedDaily, type WeatherUnits, getWindArrowRotation } from './types';
 
 	interface Props {
 		daily: FetchedDaily | null;
@@ -14,8 +16,6 @@
 
 	let { daily, selectedDay, units, onSelectDay }: Props = $props();
 
-	const today = new Date();
-
 	function getDaylightSeconds(index: number): number {
 		if (!daily) return 0;
 		const sunriseTs = daily.daily.sunrise[index];
@@ -43,7 +43,7 @@
 	<div class="flex gap-1 overflow-x-auto pb-1" style="scrollbar-width: thin">
 		{#if daily}
 			{#each daily.dailyDates as time, index (index)}
-				{@const selected = time.getDate() === selectedDay.getDate()}
+				{@const selected = isSameDayInZone(time, selectedDay, daily.timezone)}
 				{@const tempMax = daily.daily.temperature_2m_max[index]}
 				{@const tempMin = daily.daily.temperature_2m_min[index]}
 				{@const wCode = daily.daily.weather_code[index]}
@@ -68,10 +68,10 @@
 					>
 						<!-- Day label -->
 						<span class="text-sm font-bold tracking-wide">
-							{time.toLocaleDateString('en-GB', { weekday: 'short' }).toUpperCase()}
+							{formatZoned(time, daily.timezone, 'EEE').toUpperCase()}
 						</span>
 						<span class="text-[11px] text-muted-foreground">
-							{getDayLabel(time, today)}
+							{getRelativeDayLabel(time, daily.timezone)}
 						</span>
 
 						<!-- Weather icon -->

src/routes/weather/week/[location]/HourlyTable.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-	import { pad } from '$lib/utils/index';
+	import { formatUtcOffset, formatZoned, getZonedHour, isSameDayInZone } from '$lib/utils/date';
 
 	import { getTempStyle } from '../../utils/colors';
 	import weatherCodes from '../../utils/weather-codes';
@@ -10,8 +10,7 @@
 		getPrecipUnit,
 		getTempUnit,
 		getWindArrowRotation,
-		getWindUnit,
-		isCurrentHour
+		getWindUnit
 	} from './types';
 
 	interface Props {
@@ -54,36 +53,31 @@
 	}
 
 	function timeToFraction(date: Date): number {
-		const totalMinutes = date.getHours() * 60 + date.getMinutes();
-		const firstMin = cellData[0].date.getHours() * 60;
+		const tz = data.timezone;
+		const hour = getZonedHour(date, tz);
+		const minutes = parseInt(formatZoned(date, tz, 'mm'), 10);
+		const totalMinutes = hour * 60 + minutes;
+
+		const firstHour = getZonedHour(cellData[0].date, tz);
+		const firstMin = firstHour * 60;
+
 		const step = is3h ? 3 : 1;
-		const lastMin = cellData[cellData.length - 1].date.getHours() * 60 + step * 60;
+		const lastHour = getZonedHour(cellData[cellData.length - 1].date, tz);
+		const lastMin = lastHour * 60 + step * 60;
+
 		const range = lastMin - firstMin;
 		if (range <= 0) return 0;
 		return Math.max(0, Math.min(1, (totalMinutes - firstMin) / range));
 	}
 
 	function formatTime(date: Date): string {
-		return `${pad(date.getHours())}:${pad(date.getMinutes())}`;
+		return formatZoned(date, data.timezone, 'HH:mm');
 	}
 
-	function formatTimezone(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);
-		return minutes === 0 ? `UTC${sign}${hours}` : `UTC${sign}${hours}:${pad(minutes)}`;
-	}
-
-	let timezoneLabel = $derived(formatTimezone(data.utc_offset_seconds));
+	let timezoneLabel = $derived(formatUtcOffset(data.utc_offset_seconds));
 
 	function findDailyIndex(date: Date): number {
-		return daily.dailyDates.findIndex(
-			(dd) =>
-				dd.getDate() === date.getDate() &&
-				dd.getMonth() === date.getMonth() &&
-				dd.getFullYear() === date.getFullYear()
-		);
+		return daily.dailyDates.findIndex((dd) => isSameDayInZone(dd, date, data.timezone));
 	}
 
 	function isDaytime(hourDate: Date): boolean {
@@ -97,13 +91,9 @@
 	}
 
 	function getDayIndices(dates: Date[], day: Date): number[] {
+		const tz = data.timezone;
 		return dates.reduce<number[]>((acc, d, i) => {
-			if (
-				d.getDate() === day.getDate() &&
-				d.getMonth() === day.getMonth() &&
-				d.getFullYear() === day.getFullYear() &&
-				(hourlyInterval === 1 || d.getHours() % 3 === 0)
-			) {
+			if (isSameDayInZone(d, day, tz) && (hourlyInterval === 1 || getZonedHour(d, tz) % 3 === 0)) {
 				acc.push(i);
 			}
 			return acc;
@@ -148,12 +138,19 @@
 	let daytimeFlags = $derived(dayIdx.map((idx) => isDaytime(data.hourlyDates[idx])));
 
 	let cellData = $derived(
-		dayIdx.map((idx, i) => ({
-			idx,
-			date: data.hourlyDates[idx],
-			isNow: isCurrentHour(data.hourlyDates[idx], today),
-			isDaytime: daytimeFlags[i]
-		}))
+		dayIdx.map((idx, i) => {
+			const date = data.hourlyDates[idx];
+			const tz = data.timezone;
+			const isNow =
+				formatZoned(date, tz, 'yyyy-MM-dd HH') === formatZoned(today, tz, 'yyyy-MM-dd HH');
+
+			return {
+				idx,
+				date,
+				isNow,
+				isDaytime: daytimeFlags[i]
+			};
+		})
 	);
 
 	let sunrisePercent = $derived(
@@ -195,7 +192,7 @@
 <!-- Header -->
 <div class="mb-2 flex flex-wrap items-center justify-between gap-2">
 	<h3 class="text-lg font-bold">
-		{selectedDay.toLocaleDateString('en-GB', { weekday: 'long' })} – Hourly
+		{formatZoned(selectedDay, data.timezone, 'EEEE')} – Hourly
 		<span class="ml-1 text-xs font-normal text-muted-foreground">({timezoneLabel})</span>
 	</h3>
 	<div class="flex items-center gap-1.5 text-[13px] font-semibold">
@@ -293,10 +290,12 @@
 								style="left:{leftPct}%;width:{widthPct}%"
 							>
 								{#if is3h}
-									{pad(cell.date.getHours())}
+									{formatZoned(cell.date, data.timezone, 'HH')}
 								{:else}
 									<span class="inline-flex items-baseline gap-1">
-										<span class="text-[11px] font-semibold">{pad(cell.date.getHours())}</span>
+										<span class="text-[11px] font-semibold"
+											>{formatZoned(cell.date, data.timezone, 'HH')}</span
+										>
 										<sup
 											class="text-[9px] font-semibold text-muted-foreground leading-none align-baseline"
 											>00</sup

src/routes/weather/week/[location]/MeteogramCharts.svelte

@@ -3,8 +3,8 @@
 
 	import * as echarts from 'echarts';
 
+	import { formatZoned, getRelativeDayLabel } from '$lib/utils/date';
 	import { buildCurrentTimeSeries, buildDaylightSeries, getThemeColors } from '$lib/utils/echarts';
-	import { pad } from '$lib/utils/index';
 
 	import { ChartContainer, ChartToolbar, EChart } from '$lib/components/charts';
 	import '$lib/components/charts/echarts.css';
@@ -13,7 +13,6 @@
 	import {
 		type FetchedHourly,
 		type WeatherUnits,
-		getDayLabel,
 		getPrecipUnit,
 		getTempUnit,
 		getWindDirectionLabel,
@@ -32,7 +31,6 @@
 
 	const CHART_GROUP = 'week-meteogram';
 	const MS_PER_DAY = 24 * 3600 * 1000;
-	const today = new Date();
 
 	let showCharts = $state(false);
 	let chartComponents: EChart[] = $state([]);
@@ -40,9 +38,17 @@
 	let chartOptions: Array<Record<string, unknown>> = $state([]);
 
 	export function scrollToDay(day: Date): void {
-		if (chartInstances.length === 0) return;
+		if (chartInstances.length === 0 || !data) return;
 
-		const dayStart = new Date(day.getFullYear(), day.getMonth(), day.getDate()).getTime();
+		const tz = data.timezone;
+		const targetDayStr = formatZoned(day, tz, 'yyyy-MM-dd');
+		const firstHourIdx = data.hourlyDates.findIndex(
+			(d) => formatZoned(d, tz, 'yyyy-MM-dd') === targetDayStr
+		);
+
+		if (firstHourIdx === -1) return;
+
+		const dayStart = data.timestamps[firstHourIdx];
 		const dayEnd = dayStart + MS_PER_DAY;
 		const timestamps = data.timestamps;
 		const rangeStart = timestamps[0];
@@ -90,7 +96,7 @@
 	$effect(() => {
 		if (!data) return;
 
-		const { hourly, utc_offset_seconds, timestamps, markAreas } = data;
+		const { hourly, timestamps, markAreas } = data;
 		const colors = getThemeColors();
 		const tempUnit = getTempUnit(units);
 		const precipUnit = getPrecipUnit(units);
@@ -116,7 +122,7 @@
 
 		const annotations = (): Array<Record<string, unknown>> => {
 			const series: Array<Record<string, unknown>> = [];
-			series.push(buildCurrentTimeSeries({ utcOffsetSeconds: utc_offset_seconds }));
+			series.push(buildCurrentTimeSeries());
 			const dl = buildDaylightSeries({ markAreas });
 			if (dl) series.push(dl);
 			return series;
@@ -126,7 +132,12 @@
 			type: 'time',
 			splitLine: { show: false },
 			axisLine: { lineStyle: { color: colors.axisLine } },
-			axisLabel: { color: colors.text, hideOverlap: true, show: showLabel },
+			axisLabel: {
+				color: colors.text,
+				hideOverlap: true,
+				show: showLabel,
+				formatter: (value: number) => formatZoned(new Date(value), data.timezone, 'HH:mm')
+			},
 			axisTick: { lineStyle: { color: colors.axisLine } }
 		});
 
@@ -181,7 +192,8 @@
 
 		const formatDate = (ts: number): string => {
 			const date = new Date(ts);
-			return `<b>${date.toLocaleDateString('en-GB', { weekday: 'short', day: 'numeric', month: 'short' })} ${pad(date.getHours())}:${pad(date.getMinutes())}</b><br/>`;
+			const dateStr = formatZoned(date, data.timezone, 'EEE d MMM HH:mm');
+			return `<b>${dateStr}</b><br/>`;
 		};
 
 		const isAnnotation = (name: string): boolean => name === 'Daylight' || name === 'Current Time';
@@ -488,11 +500,11 @@
 	<div class="detailed-charts" in:fade={{ duration: 200 }}>
 		<div class="charts-header">
 			<h3 class="charts-title">
-				{selectedDay.toLocaleDateString('en-GB', { weekday: 'long' })}
+				{formatZoned(selectedDay, data.timezone, 'EEEE')}
 				<small>
-					{getDayLabel(selectedDay, today) !==
-					selectedDay.toLocaleDateString('en-GB', { weekday: 'long' })
-						? ` (${getDayLabel(selectedDay, today)})`
+					{getRelativeDayLabel(selectedDay, data.timezone) !==
+					formatZoned(selectedDay, data.timezone, 'EEEE')
+						? ` (${getRelativeDayLabel(selectedDay, data.timezone)})`
 						: ''}
 				</small>
 			</h3>

src/routes/weather/week/[location]/SunInfo.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-	import { pad } from '$lib/utils/index';
+	import { formatZoned } from '$lib/utils/date';
 
 	import type { FetchedDaily } from './types';
 
@@ -23,19 +23,19 @@
 	});
 </script>
 
-{#if sunrise && sunset}
+{#if daily && sunrise && sunset}
 	<div class="sun-info">
 		<div class="sun-item">
 			<svg class="fill-foreground" width="24px" height="24px">
 				<use class="stroke-2" xlink:href="/images/weather-icons/wi-sunrise.svg#Layer_1"></use>
 			</svg>
-			<span>{pad(sunrise.getHours())}:{pad(sunrise.getMinutes())}</span>
+			<span>{formatZoned(sunrise, daily.timezone, 'HH:mm')}</span>
 		</div>
 		<div class="sun-item">
 			<svg class="fill-foreground" width="24px" height="24px">
 				<use class="stroke-2" xlink:href="/images/weather-icons/wi-sunset.svg#Layer_1"></use>
 			</svg>
-			<span>{pad(sunset.getHours())}:{pad(sunset.getMinutes())}</span>
+			<span>{formatZoned(sunset, daily.timezone, 'HH:mm')}</span>
 		</div>
 	</div>
 {/if}

src/routes/weather/week/[location]/types.ts

@@ -9,6 +9,7 @@ export interface WeatherUnits {
 export interface FetchedHourly {
 	hourly: WeekHourlyData;
 	utc_offset_seconds: number;
+	timezone: string;
 	timestamps: number[];
 	hourlyDates: Date[];
 	markAreas: MarkArea[];
@@ -16,6 +17,7 @@ export interface FetchedHourly {
 
 export interface FetchedDaily {
 	daily: WeekDailyData;
+	timezone: string;
 	dailyDates: Date[];
 }
 
@@ -56,25 +58,3 @@ export const getWindDirectionLabel = (deg: number): string => {
 	];
 	return dirs[Math.round(deg / 22.5) % 16];
 };
-
-export const getDayLabel = (date: Date, today: Date): string => {
-	const MS_PER_DAY = 24 * 3600 * 1000;
-	const diff = Math.round(
-		(new Date(date.getFullYear(), date.getMonth(), date.getDate()).getTime() -
-			new Date(today.getFullYear(), today.getMonth(), today.getDate()).getTime()) /
-			MS_PER_DAY
-	);
-	if (diff === 0) return 'Today';
-	if (diff === 1) return 'Tomorrow';
-	if (diff === -1) return 'Yesterday';
-	return `${date.getMonth() + 1}-${date.getDate()}`;
-};
-
-export const isCurrentHour = (date: Date, now: Date): boolean => {
-	return (
-		date.getDate() === now.getDate() &&
-		date.getMonth() === now.getMonth() &&
-		date.getFullYear() === now.getFullYear() &&
-		date.getHours() === now.getHours()
-	);
-};