With 'Heiki-ho' (the method of placing the 24 traditional seasonal divisions of a year by making equal time intervals), the doyo days of summer are calculated as the 18 days before 'risshu' (the first day of autumn), and with the recently popular 'Teiki-ho' (the method of placing the 24 traditional seasonal divisions of a year by equally dividing the ecliptic into 24 parts, calculating the date when the sun passes each of those parts and setting the seasonal divisions into the date), the doyo days of summer fall on the dates the sun passes the obliquity of 117°to 135°, and they are calculated as approximately 18 days around risshu.
夏の土用は平気法では立秋の前の18日間、近年用いられている定気法では黄道座標が117度から135度までと定義され、約18日間ということになる。