Ascendant is the sign which the eastern horizon passes through at anytime and anywhere around the world.
The zodiac which is a virtual belt consisting of 12 signs is a fictive belt around the world. The world spins in its orbit in this virtual belt in 24 hours. So the eastern horizon changes sign every two hours.
It is important to know your birth time within this correct two hour period. When you know this correct span of two hour duration, your birth chart becomes readable.
This is so because the birth time, thus the ascendant indicates which sign will correspond to your 1. house. All 11 houses will follow this one respectively, sign by sign.
The correct and decent way to look at a birth chart is to look at it using the whole sign system. Therefore, it is sufficient to know your birth time within the correct two hours span of time.