When were the 24 solar terms added to the calendar?

In Huainanzi, written by Liu An, King of Huainan in the Western Han Dynasty, there is a complete record of the 24 solar terms. The taichu calendar compiled by Luo, a folk astronomer in the Western Han Dynasty, formally defined the 24 solar terms in the calendar, and defined their astronomical positions.

Twenty-four solar terms are the "agricultural climate calendar" that farmers have always attached great importance to. Since the Western Han Dynasty, the twenty-four solar terms have been used for generations to guide agricultural production, arrange farming according to the solar terms, and carry out farming activities such as sowing, field management and harvesting.

Because the lunar calendar in China is based on the movement of the sun and the moon, it can't fully reflect the solar cycle. China is an agricultural society, agriculture needs to know the movement of the sun strictly, and farming is carried out entirely according to the sun, so the "twenty-four solar terms" that reflect the solar cycle alone are added to the calendar as the standard for determining leap months.