《吕氏春秋·纪·季夏纪》
迁移一舍要经过七颗星，一颗星代表一年，三七二十一年，所以我说 您可延寿二十一年 。
舍行七星，星一徙当一年，三七二十一，臣故曰 君延年二十一岁 矣。
Moving one she requires seven stars, with each star representing one year, three times seven is 21 years, that is why I said you will have 21 years added to your life.