There are tecniques for rectification of birth times, and also expensive software that will give you the most probable time BUT all of them work better the more accurate the approximation to begin with. Thus if you know your birth time withina half an hour, you're more likely to get something accurate than if you only know it to within 2 hours.
That requires you to do at least some detective work. Getting a copy of your birth certificate is the best thing you can do, even if the time is rounded, you then have a good approximation to work on. Failing that, the recollections of aunts/uncles and friends of your parents at the time will be the best you can do. Don't expect anything accurate in the latter cases but you should be able to get something like 'very early morning' or 'just before lunch' or 'sometime in the evening'
You can then try some techniques to narrow down still further but I'm afraid that you will always have some element of error in the timing.
Incidentally, if we are being nit picking on timing, ANY recorded time on a birth certificate will be subject to error, though hopefully no more than a minute or two, unless you have someone with a stopwatch whose only job is to record the moment of birth and that they execute their job perfectly. Rectification on such timed births is likely to be very accurate.
However, for most modern purposes a birth time with an accuracy of 15 to 30 minutes will work reasonably well. The extremely accurate timing (whether using rectification or actual observation) is only really nescessary for predictive systems such as Primary Directions.