Glossary
Birth time
Rising and setting lines (AC/DC) move quickly, roughly a degree of chart every four minutes, so they need a reasonably exact birth time. Meridian lines (MC/IC) are steadier and shift least when the time is approximate, though they still need some time of day to be drawn at all.
When a birth time is unknown, every line depends on the time of day, so the map draws none of them rather than faking any from a noon default; the planetary positions and sign placements still compute. Narrowing the time to a rough part of the day draws the full map as a flagged best estimate instead. Birth certificates, hospital records, and family memory (in that order) are the usual sources for pinning a time down.