Birth time rectification is a skill that every serious astrologer needs to learn, since approximately 10% of charts have issues regarding their birth time. Either there is some uncertainty about the birth time, or there is no known birth time at all, or the ascendant is on a cusp between two signs, etc.
A good first step in birth time rectification is to take a logical approach in which you consider the person's character traits and then analyze their chart. For instance, I'm a tennis fan and recently was looking at the chart of Ryan Harrison, a 19 year old American who's suddenly climbed up in the rankings into the top 75 players in the world. He's a tough, spunky, highly competitive kid with a hot temper. He's often seen throwing his racquet, yelling, or challenging umpires. There is no available birth time for him, so I just chose 12 noon. Here's his chart:
Ryan Harrison's Natal Chart
Notice that at 12:00pm Harrison's Moon is at late degrees Gemini at 27 degrees 54 minutes. By 3:32pm the Moon enters Cancer. In Gemini, the Moon is conjunct Ketu and aspected by Mars, which are two fiery and temperamental planets. Gemini is an outgoing, expressive, communicative sign, whereas Cancer is an introspective, passive, and reserved sign, especially with the Moon being aspected by Saturn. Given this clear distinction between the Moon placed in Gemini versus Cancer, it makes logical sense that he was born before 3:32pm. That's a nice short cut that didn't take much time. Now instead of considering a full 24 hour period, we've cut it down to a 15 1/2 hr period.
Now, let's consider the ascendants. At 12:00am on May 7, 1992 in Shreveport Louisiana the ascendant was at 16 degrees Sagittarius, and at 3:32pm the ascendant was at 27 degrees Leo. So we'll just eliminate Virgo, Libra, and Scorpio as possible ascendants.
If you look at his chart with these two extreme birth times of 12am and 3:32pm, you'll see that in either case he's in a Saturn mahadasha right now. Given his fiery temperament, self-confident and cocky attitude, and his disciplined career focus I would first consider Aries ascendant as his likely ascendant. Aries ascendant runs at sunrise from 5:18am until 6:49am. This is not only the most fiery ascendant, which matches his temperament, but it would place exalted Sun (confidence) in the ascendant and Saturn (discipline) in its own sign in the 10th house of career. Given that he gave up high school and turned pro at the ripe age of 15, I would certainly expect a dasha sequence that supported that. Saturn dasha with Saturn in its own sign in the 10th house forming a shasha mahapurusha yoga gives this. Basketball superstar Michael Jordan also had this combination of an Aries ascendant with Saturn in the 10th house.
Of course, these are only quick initial steps in birth time rectification and nothing definitive has been established about Harrison's real birth time. I've just presented this article to emphasize taking an initial rational approach based on known character traits and known facts/events about a person's life. You can see how in a short period of time we narrowed the range of his birth time down to a more manageable level. Then we honed in on an ascendant that made the most logical sense. However, to do a complete birth rectification process is a much more complicated and time-consuming task that involves analyzing 15-20 events in a person's life and making sure that they all make logical sense, not only in the natal chart, but in all the relevant divisional charts. In addition, life events of family members should also be verified with the rectified birth time. That's a process than can take several hours, whereas we reached a logical ball park range of time in much less time.
Watch a short video of Ryan Harrison: