《黄帝内经·素问·六节藏象论》
确定了岁首冬至日为天度年的开始，用圭表的日影确定正午的时间，随着日月的运行而推算节气的盈余，直到岁尾，整个天度的变化就可以完全计算出来了。
立端于始，表正于中，推余于终，而天度毕矣。
By taking the winter solstice to be the beginning of the measurement of the heavens, and using the shadow cast by the sundial to determine noon, we can use the cycles of the sun and the moon to extrapolate the amount left over, and by the end of the year, we can fully calculate the changes that the heavens undergo.