If the birth time cannot be obtained, or is not precise, some astrologers try to find it by rectification. This is usually done by making a list of important events in the person's life, and calculating what birth time would give the most appropriate progressions and/or transits for these dates. Another method, often combined with the former, is to decide what birth time would give a chart that seems to describe the person most accurately - especially as regards physical appearance. Not all astrologers approve of rectification, however.
Read more about this topic: Natal Chart