fix: use explicit Eastern timezone for day boundary instead of system TZ

getTodayLocal() relied on system clock hours, which broke in the web
container (TZ defaulting to UTC) — the day flipped at 11 PM EDT (3 AM
UTC) instead of 3 AM Eastern. Now uses Intl.DateTimeFormat with an
explicit America/New_York timezone. Also replaced all toISOString()
date formatting with local-component helpers to avoid UTC conversion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/page.tsx

@@ -1,5 +1,5 @@
 import { HomePageClient } from "@/components/HomePageClient";
-import { getTodayLocal } from "@/lib/env";
+import { getTodayLocal, formatDateLocal } from "@/lib/env";
 
 const BACKEND_URL = process.env.BACKEND_URL ?? "http://localhost:3001";
 
@@ -12,13 +12,13 @@ function getWeekStart(param: string | undefined): string {
     const d = new Date(param + "T00:00:00");
     if (!isNaN(d.getTime())) {
       d.setDate(d.getDate() - d.getDay());
-      return d.toISOString().slice(0, 10);
+      return formatDateLocal(d);
     }
   }
   const todayIso = getTodayLocal();
   const d = new Date(todayIso + "T00:00:00");
   d.setDate(d.getDate() - d.getDay());
-  return d.toISOString().slice(0, 10);
+  return formatDateLocal(d);
 }
 
 export default async function HomePage({ searchParams }: PageProps) {

backend/src/db/queries.ts

@@ -1,5 +1,6 @@
 import type Database from "better-sqlite3";
 import { getDb } from "./index";
+import { getTodayLocal } from "../../../lib/env";
 
 import type { DayData } from "../../../lib/types";
 export type { DayData };
@@ -126,7 +127,7 @@ export function isMonthScraped(
 ): boolean {
   const daysInMonth = new Date(year, month, 0).getDate();
   const lastDay = `${year}-${String(month).padStart(2, "0")}-${String(daysInMonth).padStart(2, "0")}`;
-  const today = new Date().toISOString().slice(0, 10);
+  const today = getTodayLocal();
 
   if (lastDay < today) return true;
 

backend/src/routes/calendar.ts

@@ -2,7 +2,7 @@ import { Hono } from "hono";
 import { PARKS } from "../../../lib/parks";
 import { QUEUE_TIMES_IDS } from "../../../lib/queue-times-map";
 import { getCoasterSet } from "../../../lib/coaster-data";
-import { getTodayLocal, isWithinOperatingWindow, getOperatingStatus } from "../../../lib/env";
+import { getTodayLocal, formatDateLocal, isWithinOperatingWindow, getOperatingStatus } from "../../../lib/env";
 import { fetchToday } from "../../../lib/scrapers/sixflags";
 import { fetchLiveRides } from "../../../lib/scrapers/queuetimes";
 import { getDateRange, getParkMonthData, type DayData } from "../db/queries";
@@ -22,7 +22,7 @@ app.get("/week", async (c) => {
   const weekDates = Array.from({ length: 7 }, (_, i) => {
     const d = new Date(startParam + "T00:00:00");
     d.setDate(d.getDate() + i);
-    return d.toISOString().slice(0, 10);
+    return formatDateLocal(d);
   });
   const endDate = weekDates[6];
   const today = getTodayLocal();
@@ -53,7 +53,7 @@ app.get("/week", async (c) => {
   const currentWeekStart = (() => {
     const d = new Date(today + "T00:00:00");
     d.setDate(d.getDate() - d.getDay());
-    return d.toISOString().slice(0, 10);
+    return formatDateLocal(d);
   })();
   const isCurrentWeek = startParam === currentWeekStart;
 

components/WeekNav.tsx

@@ -25,10 +25,17 @@ function formatLabel(dates: string[]): string {
   return `${startStr} – ${endStr}`;
 }
 
+function formatDateLocal(d: Date): string {
+  const y = d.getFullYear();
+  const m = String(d.getMonth() + 1).padStart(2, "0");
+  const day = String(d.getDate()).padStart(2, "0");
+  return `${y}-${m}-${day}`;
+}
+
 function shiftWeek(weekStart: string, delta: number): string {
   const d = new Date(weekStart + "T00:00:00");
   d.setDate(d.getDate() + delta * 7);
-  return d.toISOString().slice(0, 10);
+  return formatDateLocal(d);
 }
 
 export function WeekNav({ weekStart, weekDates, isCurrentWeek }: WeekNavProps) {

docker-compose.yml

@@ -6,6 +6,7 @@ services:
     environment:
       - NODE_ENV=production
       - BACKEND_URL=http://backend:3001
+      - TZ=America/New_York
     restart: unless-stopped
 
   backend:

lib/env.ts

@@ -12,26 +12,54 @@ export function parseStalenessHours(envVar: string | undefined, defaultHours: nu
   return Number.isFinite(parsed) && parsed > 0 ? parsed : defaultHours;
 }
 
+const APP_TIMEZONE = "America/New_York";
+
 /**
- * Returns today's date as YYYY-MM-DD using local wall-clock time with a 3 AM
- * switchover. Before 3 AM local time we still consider it "yesterday", so the
- * calendar doesn't flip to the next day at midnight while people are still out
- * at the park.
- *
- * Important: `new Date().toISOString()` returns UTC, which causes the date to
- * advance at 8 PM EDT (UTC-4) or 7 PM EST (UTC-5) — too early. This helper
- * corrects that by using local year/month/day components and rolling back one
- * day when the local hour is before 3.
+ * Returns today's date as YYYY-MM-DD in Eastern time with a 3 AM switchover.
+ * Uses Intl.DateTimeFormat so it works regardless of the system/container TZ.
  */
 export function getTodayLocal(): string {
   const now = new Date();
-  if (now.getHours() < 3) {
-    now.setDate(now.getDate() - 1);
+  const fmt = new Intl.DateTimeFormat("en-US", {
+    timeZone: APP_TIMEZONE,
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+    hour: "numeric",
+    hour12: false,
+  });
+  const parts = fmt.formatToParts(now);
+  const hour = parseInt(parts.find((p) => p.type === "hour")!.value, 10) % 24;
+
+  const target = hour < 3 ? new Date(now.getTime() - 86_400_000) : now;
+  return formatDateTZ(target, APP_TIMEZONE);
 }
-  const y = now.getFullYear();
-  const m = String(now.getMonth() + 1).padStart(2, "0");
-  const d = String(now.getDate()).padStart(2, "0");
-  return `${y}-${m}-${d}`;
+
+/**
+ * Format a Date as YYYY-MM-DD in a specific IANA timezone.
+ */
+export function formatDateTZ(d: Date, tz: string): string {
+  const parts = new Intl.DateTimeFormat("en-US", {
+    timeZone: tz,
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+  }).formatToParts(d);
+  const y = parts.find((p) => p.type === "year")!.value;
+  const m = parts.find((p) => p.type === "month")!.value;
+  const day = parts.find((p) => p.type === "day")!.value;
+  return `${y}-${m}-${day}`;
+}
+
+/**
+ * Format a Date as YYYY-MM-DD using its local (system-timezone) components.
+ * Use this instead of d.toISOString().slice(0,10) which converts to UTC.
+ */
+export function formatDateLocal(d: Date): string {
+  const y = d.getFullYear();
+  const m = String(d.getMonth() + 1).padStart(2, "0");
+  const day = String(d.getDate()).padStart(2, "0");
+  return `${y}-${m}-${day}`;
 }
 
 /**