Birth chart
A birth chart is a map of the sky as it stood from your birthplace at the moment you were born, showing the planets by sign and house and the aspects between them.
Three inputs decide it: date, exact time and place. The date fixes the planets, the place and time together fix the Ascendant and the house division, and only the Moon and the angles change fast enough for a few minutes to matter much.
Without a birth time the planets and most aspects survive, while the houses, the Ascendant and an exact Moon do not. That is a real limitation and not a small one, which is why a chart cast without a time should say so rather than quietly guess noon.
Frequently asked
What do I need to cast a birth chart?
Your date of birth, your birth time as precisely as you can get it, and the city you were born in.
How accurate does the birth time need to be?
Within a few minutes is comfortable. An hour out can move the Ascendant a whole sign and shift every house with it.