diff --git a/src/config/env.ts b/src/config/env.ts
index 402f6b7..55312be 100644
--- a/src/config/env.ts
+++ b/src/config/env.ts
@@ -4,6 +4,20 @@ dotenv.config();
 // Set timezone for Date operations — all attendance/time records are in Czech local time
 process.env.TZ = process.env.TZ || 'Europe/Prague';
 
+// Override Date.toJSON to serialize as local time instead of UTC
+// MySQL DATETIME stores local time, Prisma creates Date objects,
+// JSON.stringify calls toJSON() which defaults to toISOString() (UTC with Z suffix).
+// This causes times to shift by timezone offset on the frontend.
+Date.prototype.toJSON = function () {
+  const y = this.getFullYear();
+  const m = String(this.getMonth() + 1).padStart(2, '0');
+  const d = String(this.getDate()).padStart(2, '0');
+  const h = String(this.getHours()).padStart(2, '0');
+  const min = String(this.getMinutes()).padStart(2, '0');
+  const s = String(this.getSeconds()).padStart(2, '0');
+  return `${y}-${m}-${d}T${h}:${min}:${s}`;
+};
+
 function required(key: string): string {
   const val = process.env[key];
   if (!val) throw new Error(`Missing required env variable: ${key}`);