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>

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 y = now.getFullYear();
-  const m = String(now.getMonth() + 1).padStart(2, "0");
-  const d = String(now.getDate()).padStart(2, "0");
-  return `${y}-${m}-${d}`;
+  const fmt = new Intl.DateTimeFormat("en-US", {
+    timeZone: APP_TIMEZONE,
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+    hour: "numeric",
+    hour12: false,
+  });
+  const parts = fmt.formatToParts(now);
+  const hour = parseInt(parts.find((p) => p.type === "hour")!.value, 10) % 24;
+
+  const target = hour < 3 ? new Date(now.getTime() - 86_400_000) : now;
+  return formatDateTZ(target, APP_TIMEZONE);
+}
+
+/**
+ * Format a Date as YYYY-MM-DD in a specific IANA timezone.
+ */
+export function formatDateTZ(d: Date, tz: string): string {
+  const parts = new Intl.DateTimeFormat("en-US", {
+    timeZone: tz,
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+  }).formatToParts(d);
+  const y = parts.find((p) => p.type === "year")!.value;
+  const m = parts.find((p) => p.type === "month")!.value;
+  const day = parts.find((p) => p.type === "day")!.value;
+  return `${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}`;
 }
 
 /**