The rising sign and the houses are calculated from the birth time. Because the rising sign changes about every two hours, the more exact the time, the more accurate the chart.
You can find your time on your birth certificate, in hospital records or from your family. If you don't know it, tick the box below the time field; the planets are still calculated, only the houses and the rising sign are left out.
If you have no idea of your time, try rectification: it finds the most likely birth time from the dates of major events in your life.