The most typical error when making a natal chart is to insert a wrong time of birth, you should use the solar time, not the civil time (the one pictured on clocks in the moment you were born).

But... what is the Solar Time?

Just picture a vertical stick in an open ground, the Sun moving high in the Sky, then the stick will project a shadow that will be variable over time, and when the Sun is in its highest point in the Sky, the stick's shadow will be the shortest possible that day, and in that precise moment it's the Solar Midday, this is the 12:00 according to the Sun, if you have a look at your clock then very likely it will picture a different time.

As Astrology studies the relation between what happen in the Sky and in the Earth, it doesn't care about man made clocks, just about the Sun clock, this is, the relative position of the Sun to you in the moment you were born.

Bearing this in mind is not a big deal to calculate your solar time of birth if you know the civil time and the place you were born, just follow this steps:

• Find your birth place in a map which shows latitude and longitude in their margins, these are those little numbers close to that thin lines that make squares over the map, you are interested in the horizontal margin numbers (the longitude).
• Note the longitude of the place you were born, for example, in New York it would be about 73 degrees West.
• Each 15 degrees West mean solar midday will be one hour later than in Greenwich (the place in England where solar time and civil time are always the same), so making some easy calculations you get that in New York solar midday will be 4 hours and 52 minutes later than in Greenwich:
  

  73 degrees = 60 degrees + 13 degrees = 4 hours + 52 minutes
  (1 degree equals 4 minutes)

  In other words, after the Sun is in its highest point in England, you, in New York, will have to wait 4 hours and 52 minutes to have it at such point there.

• Now let's think about man made clocks, when clocks in England mark midday (12:00), what do they say in New York? they say 7:00 am, so the official time difference is 5 hours.
• If you subtract 4 hours 52 minutes from 5 hours, you have just an 8 minutes difference between solar and civil hours, that is, suppose you were born in New York at 15:45 civil time, your solar time of birth would be 15:37, because the Sun is 8 minutes ahead of the man made clocks in that city.

It's harder to write it than to understand it, just keep imagining where is the Sun in Greenwich (South East UK) and in the place where you were born.

And keep this also in mind:

• When getting the time difference you need to take into account if the DST (Daylight Saving Time) was active when you were born, this is not so easy because has changed a lot during years and places, so you need to do some research about it.
• The difference between civil and solar time can vary wildly, in the New York example is almost nothing, but sometimes it can be up to 2 or even 3 hours.
• Anyway, when calculating chinese natal charts, the time is important in 2 hour intervals, in practice is the same to have born at 3:15 am or at 4:42 am for example. So if you don't feel like bothering with this, just try the time range before and after your civil time and see what makes more sense.
• The civil time you think it was when you were born may not be the right one, is common that doctors and hospitals doesn't care too much about it, they may write "12:00" in the papers when it really was 11:13, better to ask your mother or somebody who was present then.

I hear you ask "why to do all of this if you can get your solar time calculated automatically by a computer?", the answer is that to be sure the computer do it well, because specially regarding DST the databases are not very consistent at all, and is not so strange to find errors in the longitude databases of cities, once I found a program that situated Washington DC in the West Coast of USA!

Even time differences have changed during years, you can't think that because today the time difference between London and New York is 5 hours it has always been that, there are places like Argentina where time differences and DST use has changed randomly each few years, even from one province to other.

So if you really want to be sure do some research, is fun!