BaZi actually uses the Chinese Solar Calendar (节气), not the lunar calendar most people associate with Chinese New Year. The solar year is divided into 24 segments based on the Sun's longitude — the same astronomical position the Gregorian calendar tracks.
This is why BaZi "months" begin around February 4th (Start of Spring), not February 14th or the lunar New Year. The system is solar-based and astronomically precise. You simply provide your Gregorian birth date; our system converts it automatically.