This is one of the most common confusions. The answer: by the solar term "Start of Spring" (立春), not the lunar New Year.
For BaZi purposes, the zodiac year changes at Start of Spring, which falls around February 3rd–5th each year. So a baby born on February 1st, 2024 (before Start of Spring) belongs to the previous zodiac year (Rabbit), not the lunar New Year's Dragon.
The lunar New Year (Spring Festival) is a cultural celebration, not the astronomical marker BaZi uses. Always check the exact Start of Spring date for the birth year when determining the Year Pillar.
Any BaZi calculator worth its code handles this automatically, but it's good to know the reason.