Why does the twelfth lunar month never leap?

The leap month in the lunar calendar is related to the twenty-four solar terms.

The twenty-four solar terms are based on the earth's orbit around the sun. Twenty-four solar terms are divided into twelve "knots" and twelve "qi". Starting from beginning of spring, the singular number is "solar terms" and the even number is "neutral gas". According to the calendar, every lunar month should be equipped with a "neutral gas" and a "solar term". But because the days between the two "neutral gases" are a little over 30 days, and a new moon (that is, a month in the lunar calendar) is only a little over 29 days, the difference between them is nearly one day. Therefore, "Zhongqi" will be postponed month by month in the lunar calendar, which will delay it. In this way, there is a month without "neutral gas" in the middle, so the calendar stipulates that this month without "neutral gas" is regarded as a leap month. Therefore, in the first month of the year, starting from beginning of spring, this month will never be without "neutrality". Similarly, at the end of the twelfth lunar month, starting from the slight cold, this month will never be without "neutralization", so there will never be a leap between the first month of the year and the twelfth lunar month of the year.