Fix SSR/hydration timezone flip in event date and time display (#95)

## Summary

- Adds `useHasMounted` hook (using `useSyncExternalStore` — no extra render cycle) to gate UTC timestamp formatting to client-only
- Fixes the visible time-flip in `EventSchedule` and `UpcomingCalendarPanel` where SSR rendered UTC time (e.g. "9:00 PM") and the client re-rendered with local time (e.g. "2:00 PM PDT") after hydration, with `suppressHydrationWarning` silently hiding the mismatch
- Fixes `formatEventDate` in `date-utils.ts` (and `EventSchedule`'s private copy) to parse `YYYY-MM-DD` strings as local midnight instead of UTC midnight, correcting an off-by-one day bug for UTC-negative users across all callers

## Test plan

- [ ] Navigate to a sports season page — confirm event times show in local timezone with no flash/flip on load
- [ ] Hard-reload the page — confirm no brief UTC time visible before settling on local time
- [ ] Check the home page `UpcomingCalendarPanel` — confirm same behavior
- [ ] Verify dates display correctly (e.g. no "Jun 17" showing for a "Jun 18" event near midnight UTC)

Co-authored-by: Chris Parsons <chrisparsons1127@gmail.com>
Reviewed-on: #95

app/components/sport-season/EventSchedule.tsx

@@ -1,4 +1,5 @@
-import { format, parseISO } from "date-fns";
+import { format } from "date-fns";
+import { useHasMounted } from "~/hooks/useHasMounted";
 import { Link } from "react-router";
 import { Card, CardContent, CardHeader, CardTitle } from "~/components/ui/card";
 import { Badge } from "~/components/ui/badge";
@@ -25,28 +26,13 @@ interface EventScheduleProps {
 function formatEventDate(dateStr: string | null | undefined): string {
   if (!dateStr) return "Date TBD";
   try {
-    // eventDate is stored as a date string "YYYY-MM-DD"
-    return format(parseISO(dateStr), "MMM d, yyyy");
+    const [year, month, day] = dateStr.split("-").map(Number);
+    return format(new Date(year, month - 1, day), "MMM d, yyyy");
   } catch {
     return dateStr;
   }
 }
 
-/**
- * Returns the display date for an event.
- * Prefers eventStartsAt (a full UTC timestamp → correct local date) over
- * eventDate (the UTC calendar date, which is off by one day for late-night
- * events in UTC-negative timezones like PDT).
- */
-function getDisplayDate(event: Pick<ScheduleEvent, "eventDate" | "eventStartsAt">): string {
-  if (event.eventStartsAt) {
-    const d = new Date(event.eventStartsAt);
-    if (!isNaN(d.getTime())) {
-      return format(d, "MMM d, yyyy");
-    }
-  }
-  return formatEventDate(event.eventDate);
-}
 
 function getEventTypeLabel(eventType: string): string {
   switch (eventType) {
@@ -64,6 +50,16 @@ function getEventTypeLabel(eventType: string): string {
 }
 
 export function EventSchedule({ upcomingEvents, recentEvents, leagueId, sportsSeasonId }: EventScheduleProps) {
+  const isMounted = useHasMounted();
+
+  function getDisplayDate(event: Pick<ScheduleEvent, "eventDate" | "eventStartsAt">): string {
+    if (isMounted && event.eventStartsAt) {
+      const d = new Date(event.eventStartsAt as string);
+      if (!isNaN(d.getTime())) return format(d, "MMM d, yyyy");
+    }
+    return formatEventDate(event.eventDate);
+  }
+
   const hasUpcoming = upcomingEvents.length > 0;
   const hasRecent = recentEvents.length > 0;
 
@@ -112,11 +108,9 @@ export function EventSchedule({ upcomingEvents, recentEvents, leagueId, sportsSe
                     </p>
                     <p className="text-xs text-muted-foreground flex items-center gap-1 mt-0.5">
                       <Calendar className="h-3 w-3" />
-                      <span suppressHydrationWarning>{getDisplayDate(event)}</span>
-                      {event.eventStartsAt && (
-                        <span suppressHydrationWarning>
-                          · {format(new Date(event.eventStartsAt), "h:mm a")}
-                        </span>
+                      <span>{getDisplayDate(event)}</span>
+                      {isMounted && event.eventStartsAt && (
+                        <span>· {format(new Date(event.eventStartsAt), "h:mm a")}</span>
                       )}
                     </p>
                   </div>
@@ -178,7 +172,7 @@ export function EventSchedule({ upcomingEvents, recentEvents, leagueId, sportsSe
                     </p>
                     <p className="text-xs text-muted-foreground flex items-center gap-1 mt-0.5">
                       <Calendar className="h-3 w-3" />
-                      <span suppressHydrationWarning>{getDisplayDate(event)}</span>
+                      <span>{getDisplayDate(event)}</span>
                     </p>
                   </div>
                   {event.eventType !== "schedule_event" && (

app/components/sport-season/UpcomingCalendarPanel.tsx

@@ -1,6 +1,7 @@
 import { format } from "date-fns";
 import { Calendar, ChevronRight, Users } from "lucide-react";
 import { useState, useEffect } from "react";
+import { useHasMounted } from "~/hooks/useHasMounted";
 import { Link } from "react-router";
 import { Badge } from "~/components/ui/badge";
 import {
@@ -58,9 +59,10 @@ function ParticipantList({ participants }: { participants: Array<{ id: string; n
 }
 
 function EventRow({ event, showLeague }: { event: CalendarPanelEvent; showLeague: boolean }) {
+  const isMounted = useHasMounted();
   // Prefer game-level scheduledAt over event-level eventDate for display
   const gameDate = event.earliestGameTime ? new Date(event.earliestGameTime) : null;
-  const dateStr = gameDate
+  const dateStr = isMounted && gameDate
     ? format(gameDate, "MMM d")
     : formatEventDate(event.eventDate);
   const participantCount = event.relevantParticipants.length;
@@ -73,14 +75,11 @@ function EventRow({ event, showLeague }: { event: CalendarPanelEvent; showLeague
     <div className="flex items-start gap-3 py-2.5 border-b last:border-0 group">
       {/* Date pill */}
       <div className="w-14 shrink-0 text-center">
-        <span className="text-xs font-semibold text-electric" suppressHydrationWarning>
+        <span className="text-xs font-semibold text-electric">
           {dateStr ?? "TBD"}
         </span>
-        {gameDate && (
-          <p
-            className="text-xs text-muted-foreground leading-tight"
-            suppressHydrationWarning
-          >
+        {isMounted && gameDate && (
+          <p className="text-xs text-muted-foreground leading-tight">
             {gameDate.toLocaleTimeString(undefined, {
               hour: "numeric",
               minute: "2-digit",

app/hooks/useHasMounted.ts

@@ -0,0 +1,7 @@
+import { useSyncExternalStore } from "react";
+
+const subscribe = () => () => {};
+
+export function useHasMounted(): boolean {
+  return useSyncExternalStore(subscribe, () => true, () => false);
+}

app/lib/date-utils.ts

@@ -16,7 +16,7 @@
  * clear the field. Always write the result to a separate hidden input, not back to
  * the datetime-local input itself.
  */
-import { format, parseISO } from "date-fns";
+import { format } from "date-fns";
 
 /**
  * Format a YYYY-MM-DD date string for display (e.g. "Apr 9").
@@ -25,7 +25,8 @@ import { format, parseISO } from "date-fns";
 export function formatEventDate(dateStr: string | null | undefined): string | null {
   if (!dateStr) return null;
   try {
-    return format(parseISO(dateStr), "MMM d");
+    const [year, month, day] = dateStr.split("-").map(Number);
+    return format(new Date(year, month - 1, day), "MMM d");
   } catch {
     return null;
   }