fix(datetime): local datetime parsing

frontend/src/pages/Profile.tsx

@@ -3,6 +3,7 @@ import { Eye, EyeOff, Copy, Check } from "lucide-react";
 import Box from "../components/Box.tsx";
 import { getUser } from "../utils/endpoints.ts";
 import type { User } from "../utils/types.ts";
+import { formatLocalDate } from "../utils/utils.ts";
 
 const Profile = () => {
     const [user, setUser] = useState<User | null>(null);
@@ -44,7 +45,7 @@ const Profile = () => {
                     <p className="text-sm text-gray-500">@{user.username}</p>
                     <p className="text-sm text-gray-500">{user.email}</p>
                     <p className="text-xs text-gray-400 mt-1">
-                        Member since {new Date(user.createdAt).toLocaleDateString(undefined, { year: "numeric", month: "long", day: "numeric" })}
+                        Member since {formatLocalDate(user.createdAt)}
                     </p>
                 </div>
             </Box>

frontend/src/utils/utils.ts

@@ -7,13 +7,20 @@ function formatTime(seconds: number): string {
     return `${m}:${pad(s)}`;
 }
 
+// Parses a UTC date string, appending Z if no timezone offset is present so
+// the browser always interprets the value as UTC rather than local time.
 function stringToDate(dateString: string): Date {
     const normalized = dateString.replace(/(\.\d{3})\d+/, "$1");
-    const date = new Date(normalized);
+    const withTz = /[Zz]$|[+-]\d{2}:\d{2}$|[+-]\d{4}$/.test(normalized) ? normalized : `${normalized}Z`;
+    const date = new Date(withTz);
     if (isNaN(date.getTime())) throw new Error("Invalid date string");
     return date;
 }
 
+function formatLocalDate(dateString: string, options?: Intl.DateTimeFormatOptions): string {
+    return stringToDate(dateString).toLocaleDateString(undefined, options ?? { year: "numeric", month: "long", day: "numeric" });
+}
+
 function dateToTimeAgo(date: Date): string {
     const secondsAgo = Math.floor((Date.now() - date.getTime()) / 1000);
     const units: [number, string][] = [
@@ -33,4 +40,4 @@ function dateToTimeAgo(date: Date): string {
     return `${s} second${s !== 1 ? 's' : ''} ago`;
 }
 
-export { formatTime, stringToDate, dateToTimeAgo };
+export { formatTime, stringToDate, dateToTimeAgo, formatLocalDate };