No previous message bug fix

apps/ai_tutor/helpers.py

@@ -19,9 +19,13 @@ async def check_user_cooldown(
     last_message_time_str = user_info["metadata"].get("last_message_time")
 
     # Convert from ISO format string to datetime object and ensure UTC timezone
-    last_message_time = datetime.fromisoformat(last_message_time_str).replace(
-        tzinfo=timezone.utc
-    )
+    try:
+        last_message_time = datetime.fromisoformat(last_message_time_str).replace(
+            tzinfo=timezone.utc
+        )
+    except Exception as e:  # this probably means the user has never sent a message before
+        return False, None
+
     current_time = datetime.fromisoformat(current_time).replace(tzinfo=timezone.utc)
 
     # Calculate the elapsed time