Please be sure to note that Chinese astrology makes use of sundial time, i.e. what the time would have been using a sundial. Whether you're doing ZWDS (the lunar calendar), or Bazi (the solar calendar), a birth time recorded by a standard hand watch, for example, will have to be converted into true solar time.
In order to obtain sundial time you have to take into account the following:
*) Subtract one hour if there was any summer time (daylight saving time), or war time, etc., in operation. (Useful info at this site.)
*) Longitude corrections, since the sundial time isn't exactly the same everywhere within the 15° time zone area. A watch showing mean solar time, yes, but not the sundial time, the true solar time.
*) The equation of time (due to the Earth's tilt and elliptical orbit around the sun). (Tables here.)
Example: Steve Jobs, February 24 1955, 19:15 (standard time) (Rodden Rating: AA); San Francisco, CA, 37N46, 122W25.
*) There was no summer time going on in February, so there is nothing to subtract so far.
*) San Francisco is at 122° 25' West of Greenwich, observing Pacific time, being eight hours behind UTC.
To convert longitude degrees and minutes into time, divide by 15: 122° 25' / 15 = 08:09:40. This is 9' 40'' more than time zone time (eight hours). The rule is:
More + West = subtract
Less + West = add
More + East = add
Less + East = subtract
Since 08:09:40 is more than time zone time (eight hours) and Jobs was born West of Greenwich, you have to subtract this difference (i.e. 9' 40'') from the recorded birth time: 19:15 - 00:09:40 = 19:05:20 (result so far).
*) The equation of time: on February 24 the sundial is approximately 00:13:21 slower than the local mean time, and you want to get to sundial time, so you'll have to subtract these 13 minutes: 19:05:20 - 00:13:21 = 18:51:59 = the time a sundial would have shown when Steve Jobs was born.
(You can also use the calculator found here, but using UTC instead of standard time. The standard birth time of Steve Jobs = 19:15:00, 24 February. UTC = 03:15:00, 25 February (adding eight hours).)
The day consists of 12 hours:
1st hour - 23:00 (the day before) - 00:59:59.99 (the day after) (the hour of the Rat)
2nd hour - 01:00 - 02:59:59.99 (the hour of the Ox)
3rd: 03 - 05 (branch-Yin/the Tiger)
4th: 05 - 07 (branch-Mao/Rabbit)
5th: 07 - 09 (Chen)
6th: 09 - 11 (Si)
7th: 11 - 13 (午)
8th: 13 - 15 (未)
9th: 15 - 17 (申)
10th: 17 - 19 (酉)
11th: 19 - 21 (戌)
12th: 21 - 22:59:59.99 (亥)
From this you can see that even though Steve Jobs' birth certificate says 19:15, he was not born in the 11th hour, but, due to corrections re true solar time, in the 10th. He has Ming in Wu within which are Lian Zhen and Tian Xiang.