Use their actual birth time — the moment they entered the world and took their first breath. This is the same rule as for biological children.
BaZi maps the energetic imprint at birth, which remains the person's chart regardless of their life circumstances. Adoption doesn't change the elemental blueprint any more than it changes DNA.
However, adoption can profoundly affect how the chart manifests. A child's Month Pillar represents their parents and upbringing environment. If the adopted family's energy is vastly different from the biological family's, the chart may express differently — similar to how a plant grows differently in different soil.
The chart stays the same. The environment shapes the expression.