When you have an appointment should go before the appointment time 10-20 minutes so that you won't have to rush because if you could possibly send the deterioration itself and others.
When should you have an appointment to go before the clock strikes 10-20 minutes so that you do not rush, because if you missed may adversely affect themselves and others.
When you have an appointment to appointment 10 - 20 minutes ahead of time so that you don't rush because if you don't keep up may be negatively to yourself and others.