When did the twenty-four solar terms begin?

By the Qin and Han Dynasties, the 24 solar terms had been completely established.

The book Huai Nan Zi has exactly the same name as the modern twenty-four solar terms.

In BC 104, the taichu calendar written by Deng Ping officially set 24 solar terms in the calendar, and defined the astronomical position of the 24 solar terms.

BC 104 (the first year of Emperor Wu of the Han Dynasty), that is, the first year of Emperor Wu of the Western Han Dynasty, was the first year of the seventh year of Emperor Wu of the Han Dynasty (the year of Emperor Wu of the Han Dynasty took four years).