How to calculate Zhouyi's one-year-old fortune telling?

Why do fortune tellers use nominal age instead of full age? What is the reason?

In ancient China, it was believed that heaven and earth collided and yin and yang were born together. Therefore, an old friend was given fate when she was pregnant. When he was pregnant and gave birth in October, his birth was not a real birthday, but the day he was born was the beginning of fate. But the ancients couldn't know the exact time of conception, so people in China always reported one year old. This custom has been used to this day, and many old people report their age by fictional age. Why is it called "empty"? It is also because there are many things in the uterus. Life is empty if it is not born, and emptiness is not real.

Moreover, fortune-telling also follows the ancient law, so some gentlemen still aim at nominal age.