How accurate does the birth time need to be?
Ideally — to the minute. The Moon changes gates every ~2 hours, the Sun — every ~5-6 days. A 15-30 minute difference can change Moon gates, planet lines, and sometimes even the Profile. If you don't know the exact time, try rectification — a method of refining birth time through life events. HD Matrix has a dedicated rectification tool.
Calculate your Human Design chart for free at hdmatrix.pro.