The calculation of the dog days starts from the third "G" day after the summer solstice. The first dog day lasts for 10 days, the middle dog days last for 10 or 20 days, and the last dog day lasts for 10 days.

The dog days of summer appear during the mild heat and extreme heat. They are the hottest, humid and sultry days of the year. The so-called "Dog Days" refers to the "Dog Days" of the lunar calendar, which is the hottest period of the year.

The length of Futian mainly lies in the difference between Zhongfu. Because the third Geng day after the summer solstice (the first fall) occurs at different times every year, the number of days leading to the middle fall is also different. The duration of the incubation period may be long or short, it may be 10 days or 20 days. So there are two situations: some years have 30 days of dog days, and some years have 40 days.