Natal chart
A snapshot of the sky at the moment and place you were born.
It needs three things: date, time, and the latitude and longitude of the birthplace. The date fixes roughly where the planets are; the time and place fix the Ascendant and how the twelve houses are cut. Without a time you still get planetary positions but no houses — so "can I get a reading without my birth time" answers to "half of one".
The natal chart underlies everything else. Transits, returns, progressions all describe how the current sky sits against the sky you were born under.
An hour's error moves the Ascendant about 15°, often into a different sign, and rotates all twelve houses with it. So the accuracy of the time decides whether the house-based half is usable at all.