From the beginning, you should learn to strengthen your sankalp shakti (power of will) by resolving that you will wake up on time and meditate foor 10, 15 or 20 minutes. The mind is the greatest of all timekeepers, and as you progress, you will find that the mind wakes you up for your meditation. Nothing external is really needed when you have decided that you want to awaken and meditate at a particular time.
Generally, it is not necessary to use an alarm clock to time a meditation itself. It’s also rather unpleasant to end a tranquil meditation with the jarring tone of alarm clock. If you are concern about time, keep the alarm clock in view so you can check the time, or better still, try to set up your meditation session so that you don’t feel so much time pressure. Meditate earlier in the morning, or in the evening when there are no more duties or responsibilities awaiting you. (i.e. do not keep your schedule too much tight. This will help you to relax and meditate properly without tension.)
Source: Extracted from “Meditation and it’s Practice” by Swami Rama (Pages 82-83)