《宋书·志·卷十三》
推算闰月的方法：将章岁减去闰余，其差除以闰法就得到月数，从周正起算并外推一月，就是闰月的月份。
推闰术：以闰余减章岁，余满闰法得一月，命以天正，算外，闰所在也。
The method to deduce an intercalary month: Subtract the intercalary remainder from the integer year. Divide the difference by the intercalary method to get the month number. Starting from the Zhouzheng, add one month and that is the month of the intercalary month.