Why is there a difference between the month I saw in some ancient almanac and the real month? For example, the book says that August is a unitary month, but the real month is only after the date 10?

The date of the constellation follows the solar terms. Only when the solar terms change will the moon change.

For example, it's July 1. However, the "light summer" passed on July 7. In the eight characters, consider entering next month.

Because it involves the advance and retreat of the five elements in a year.

So if you look at the horoscope, you must look at the 24 solar terms.