This is the establishment of the exact time of birth, which is given by fate, and not by a "tag". We are born under different circumstances: medical intervention, stress, cesarean section, etc. By setting the exact time of birth, you can answer a lot of questions about your natal chart, find the causes of certain situations. This is the "highest" Astrology.
Rectification is a complex process that takes from 1 to several days. Its speed depends on the source data