Fix: Update last servicing dates when using the date cheat (#9694)

src/vehicle.cpp

@@ -763,6 +763,16 @@ uint32 Vehicle::GetGRFID() const
 	return this->GetEngine()->GetGRFID();
 }
 
+/**
+ * Shift all dates by given interval.
+ * This is useful if the date has been modified with the cheat menu.
+ * @param interval Number of days to be added or substracted.
+ */
+void Vehicle::ShiftDates(int interval)
+{
+	this->date_of_last_service += interval;
+}
+
 /**
  * Handle the pathfinding result, especially the lost status.
  * If the vehicle is now lost and wasn't previously fire an