Why there can be two signs
The Chinese calendar has two year boundaries. Chinese New Year falls on a new moon between 21 January and 20 February. Lichun, the Start of Spring, is the solar term when the Sun reaches 315° of ecliptic longitude, every year on 3, 4 or 5 February.
The zodiac sign is traditionally counted from Chinese New Year. BaZi uses Lichun as the boundary, because its year and month pillars follow the solar calendar. People born between the two dates have one sign in the horoscope and another year in the chart, and neither calculation is wrong.
Both boundaries apply in Beijing time. If you were born late in the evening before Chinese New Year or Lichun, it may already have been the next day in Beijing. For such dates a chart with your time and place gives the exact answer.