Question: What do "Jiansheng" and "Fu Sheng" mean?

In rural marriage, there is a procedure called "sending eight characters". The man sends a "eight-character post" with the date of birth of the man to the woman's home together with the dowry.

The woman's family will fill in the year, month, day and time of the woman's birth on the "eight-character post" and the man will bring it back. Through the birthdays of both men and women, the wedding will be held on an auspicious day.

The general writing format is male left and female right, and men and women write with "year, month and day". In principle, "Jia Zi" should be used to represent the year, month and day, for example, Jia Yin was born in ugliness, and Kun was born in ugliness (Jia Zi is made up casually). However, at present, Mr. Wang is not so particular about it, and instead adopts the lunar calendar, such as: 1998120/20, and 1998/0/0/5.

Other literal meanings are understandable, but what do "Jiansheng" and "Fu Sheng" mean?

I vaguely remember hearing the old gentlemen argue and analyze when I was a child, but I don't remember the specific meaning. I also searched on Baidu, Headline and other platforms, but I still don't know.

Please ask Fang Jia, who knows this very well, to help explain.