Why birthdate can matter more than birth year
The Chinese zodiac year changes on Chinese New Year, not on January 1. Because Chinese New Year usually falls between late January and mid-February, two people born in the same Gregorian year can belong to different zodiac years if one birthday falls before the Lunar New Year boundary.
That is why the calculator supports exact birthdate mode. If you only know the year, the result is still useful, but January and early-February births may need a boundary adjustment.