Same Beijing Time of 8am means very different sun positions in Urumqi versus Harbin. BaZi uses true solar time, not Beijing Time.
True solar time adjusts Beijing Time (based on 120°E) to your actual birthplace longitude. Every degree west of 120°E subtracts 4 minutes. Every degree east adds 4 minutes.
The Hour Pillar determines children and later life. If you were born near an hour boundary (7am, 9am), Beijing Time and true solar time may differ by a full shichen, completely changing your Hour Pillar and chart reading.
True solar time = Beijing Time + (birthplace longitude - 120) × 4 minutes + equation of time correction. Guangzhou is at about 113°E, about 28 minutes behind Beijing Time. So 8:00am Beijing Time in Guangzhou is about 7:32am true solar time — still Chen hour.