Why an Exact Birthtime is Crucial
Two of the most important astrological factors (the Ascendant and Midheaven) move so quickly (approx. 15°/Hour) that, in order to have "usable" Angles of the birthchart, we must have a very accurate birthtime.
To be correct to the nearest degree (°), the birthtime must be within 2 mins of correct. In order to be correct to the nearest minute ('), the birthtime must be within 2 secs of correct.
An incorrectly timed chart propagates that error through all of the further astrological indications that are derived from that incorrect chart. The timing for events and important phases in a person's life can only be correctly seen when the birthtime is correct.
Additionally, in many charts there are Angles/House-cusps which are at the very beginning/end of a sign. Without having the correct signs, the wrong planets will be shown to be ruling those houses and false connections will be made.
In my experience, I have found that even documented birthtimes are often off by some number of minutes. An error of 10 mins in time is very normal. This 10 mins of time relates to errors in Angles and House Cusps of approximately 2.5° (or 2.5 Years of time by some prediction systems)
How to Correctly Rectify
The most accurate technique in which to get the correct birthtime is by using Topocentric Primary Directions, and the confirmation of Secondary Progressions, Progressed Sidereal Solar Returns, and Transits.
Polaris (rectification software by Isaac Starkman) searches the possible birthtime range and lists the very best birthtimes (finding best aspects to events across all events). From this list, using the other systems, we are able to confirm which birthtime is, in fact, the correct one. By using this method, we are getting the chart that works best across all of these systems.
Rectification was normally a very subjective process. Ten separate astrologers are likely to suggest ten different "birthtimes." With Polaris, the subjectivity is mostly removed, since the program is giving a direct showing of how each particular possible birthtime measures up in terms of appropriate aspects to events, across ALL events.
Why Polaris Is Important
Polaris uses Topocentric Primary Directions to look at all birthtimes and find the most likely correct birthtime. By confirmation through other systems (Secondary Progressions, Progressed Sidereal Solar Return, and Transits) the correct birthtime can be found. Each rectification is different, but generally speaking, the more events Polaris has, the more likely it is to be able to find and demonstrate the correctness of a particular birthtime.
Without Polaris, the search for the right birthtime has a needle-in-the-haystack quality. The birthtime has to be "coarsely tuned" to the approximate birthtime and then whittled down manually, in an attempt to derive THE birthtime.
Polaris takes a different approach. By taking all the times within the search range and looking at each of those moments for a best fit, gives a list of the likely birthtimes and now we must only check these birthtimes in order to find which is correct. (The other systems are usually very dramatic in this regard.)
For more information on Rectification and how I use Polaris to help find precision birthtimes, please read my article, Getting the Birthtime Right, on Donna Cunningham's Skywriter Blog.