Fix timezone issue in formatDateTime for appointments page

- Changed formatDateTime to use UTC methods instead of toLocaleString
- Prevents timezone conversion when displaying appointment times
- Now booking at 11:45 shows as 11:45 in appointments page, not 12:45
- Consistent with formatTime which already uses UTC
- Manual formatting to match previous format: 'Mon, Jan 15, 11:45'

frontend/app/utils/date.ts

@@ -25,18 +25,23 @@ export function formatTime(isoString: string): string {
 }
 
 /**
- * Format datetime from ISO string to a readable format.
+ * Format datetime from ISO string to a readable format in UTC.
+ * Uses UTC to avoid timezone conversion issues when displaying appointment times.
  */
 export function formatDateTime(isoString: string): string {
   const d = new Date(isoString);
-  return d.toLocaleString("en-US", {
-    weekday: "short",
-    month: "short",
-    day: "numeric",
-    hour: "2-digit",
-    minute: "2-digit",
-    hour12: false,
-  });
+  
+  // Use UTC methods to avoid timezone conversion
+  const weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"];
+  const months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
+  
+  const weekday = weekdays[d.getUTCDay()];
+  const month = months[d.getUTCMonth()];
+  const day = d.getUTCDate();
+  const hours = String(d.getUTCHours()).padStart(2, "0");
+  const minutes = String(d.getUTCMinutes()).padStart(2, "0");
+  
+  return `${weekday}, ${month} ${day}, ${hours}:${minutes}`;
 }
 
 /**