Birth time is the single input we get asked about most, because a lot of people simply don't have it — no birth certificate detail, a home birth with no record, a country whose paperwork doesn't include it. Here's what actually happens if you leave it out.
Your Sun sign is unaffected — it only depends on date, not time. Moon sign is usually fine too, since the Moon changes sign roughly every two and a half days, so unless you were born right at a sign boundary, the date alone is enough. What breaks is anything dependent on exact time: your Rising sign, house placements, and by extension a slice of the more granular chart detail that feeds the Soul and Intellectual fates specifically.
If you don't know your time, use midday as a neutral estimate rather than guessing a time to "match your personality" — that just introduces confirmation bias into your own chart. We flag any match involving an estimated time so you know which parts of the read to trust less.