How the calculator chooses a year
The Hong Kong Observatory’s Gregorian–Lunar conversion table marks the first day of the first lunar month for each year. A birthday before that date belongs to the preceding lunar year. We then use the twelve earthly branches for the animal and the ten heavenly stems for the element and yin or yang designation.
This uses Lunar New Year as the changeover. Some fortune-telling traditions instead change the year at the “Spring Commences” solar term (Lìchūn), usually around February 4; that convention can give a different answer. The displayed boundary follows the Observatory’s Hong Kong calendar date. Local calendar interpretations near a new moon may differ by a day.