Rectification is the practice of working backwards from a person's life to a proposed birth time. Instead of casting a chart from a known time and reading events out of it, you take the events you know and find the time that makes the chart fit them. It is the only route to houses when the recorded time is lost, and it is also the most contested technique in the tradition.
The Ascendant moves about one degree every four minutes, so a birth time is equivalent to a very precise Ascendant degree. Rectification searches the range of plausible times for the one whose angles and house cusps best match a set of dated life events. Move the time, and the whole house structure rotates with it. The right time is supposed to be the one where the events land on the right cusps.
The standard input is a list of dated events: marriages, divorces, births of children, deaths of parents, job changes, relocations, major illnesses, accidents. Traditional practice also weighs physical appearance and temperament against the rising sign, and modern practice often adds predictive techniques, checking which candidate time makes transits and progressions line up with the event dates.
Different astrologers work in different orders, but most follow a similar arc. The goal is not to find a magical match on one event, because almost any time will match something. The goal is a time that holds up across many independent events.
Expect the process to be iterative. A first pass narrows the window, a second pass tests the survivors, and the result is a proposed time with a stated range rather than a single minute.
The central objection is that rectification can always succeed. With enough free parameters, a large event list, and a practitioner choosing which techniques to apply and how tightly to judge a hit, some birth time will fit. That flexibility is exactly what makes the result hard to test. If two competent astrologers rectify the same person independently and land on different times, neither can claim to have recovered the truth.
There is also a selection effect. Rectification is usually performed on charts of people who are already famous or well documented, which means the practitioner often knows the biography intimately before starting. Knowing the answer shape makes it easier to find one that fits.
As a discipline for organizing evidence, it is useful. It forces you to enumerate what you know about a life and to be explicit about which chart features you think correspond to which events. That exercise has value even if the final time is uncertain.
As a claim of recovered fact, it should be stated with the uncertainty it carries. Say 'this time fits the events better than the alternatives' rather than 'this is your birth time.' If the recorded time later surfaces and contradicts the rectified one, the recorded time wins. That is the honest hierarchy, and it is worth stating up front to anyone considering the process.
Look for the time first. Birth certificates in many places list it, hospital records sometimes retain it, and family members occasionally remember a detail like a meal or a shift change that pins the hour. A discovered record beats a rectified guess every time, and it takes far less work.
If no record exists, decide whether you need houses at all. A great deal of chart reading does not require them, and rectification is a significant investment that returns a proposal rather than a fact. Go in knowing that, and the result will be more useful than if you expected a certificate.