The words a chart uses
Why does birth time matter in astrology?
Birth time fixes which way the horizon faced at your birth, so it sets the Ascendant, the Midheaven and the houses; without it, those parts of a chart are unknown.
The planets move slowly enough that a birth date places most of them in their signs. The Earth turns much faster: the horizon passes through every sign of the zodiac in about a day. So the time of birth decides the Ascendant, the sign rising on the eastern horizon, the Midheaven above, and from them the twelve houses.
The Moon moves too, about thirteen degrees a day, so on some dates an unknown time leaves its sign uncertain.
When you do not know the time
If you are unsure, use the time on a birth certificate or a hospital record where one exists. A time remembered by family is worth using, with the understanding that a difference of minutes can move a house cusp.
Becoming Choice lets you say you do not know the time. The chart is then calculated for midday and marked as such: no Ascendant, no Midheaven, no houses and no quadrants are shown, and the Moon is marked uncertain. The signs of the planets, their aspects and the balance of the signs are still read. Nothing is estimated and presented as known.