First of all, what you asked is wrong. It should be "when the time is good, the day will be more expensive, and the child will be born later." Then, the day is more expensive, referring to the four days of Dingyou, Dinghai, Guisi, and Guimao. "Rigui" refers to the time of birth. The corresponding earthly branches in these four days are: Youshi, Haishi, Sishi and Maoshi.