《汉书·志·律历志下》
推演正月朔：用月法乘积月，满法就得一，命名为积曰，不满积曰就命名为小余。
推正月朔，以月法乘积月，盈日法得一，名曰积日，不盈者名曰小余。
To deduce the date of the first day of the first lunar month: multiply the Method for Moon by the Method for Full, if the result is a whole number, then that day is called "ji ri"; if the result is not a whole number, then that day is called "xiao yu".