Fix dispute

backend/app/api/v1/assignments.py

@@ -1,7 +1,7 @@
 """
 Assignment details and dispute system endpoints.
 """
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from fastapi import APIRouter, HTTPException, Request
 from fastapi.responses import Response, StreamingResponse
 from sqlalchemy import select
@@ -49,7 +49,9 @@ def build_dispute_response(dispute: Dispute, current_user_id: int) -> DisputeRes
             my_vote = v.vote
             break
 
-    expires_at = dispute.created_at + timedelta(hours=DISPUTE_WINDOW_HOURS)
+    # Ensure expires_at has UTC timezone info for correct frontend parsing
+    created_at_utc = dispute.created_at.replace(tzinfo=timezone.utc) if dispute.created_at.tzinfo is None else dispute.created_at
+    expires_at = created_at_utc + timedelta(hours=DISPUTE_WINDOW_HOURS)
 
     return DisputeResponse(
         id=dispute.id,