……大王,您所期待的长恒纪元就到来了,这个恒纪元将持续三年零九个月,其间气候温暖,是一个黄金纪元。
刘慈欣,《三体》
This is from the novel The Three Body Problem. I'm a little bit puzzled as to why there is a 零 added in 三年零九个月 = "three years and nine months"; we usually we don't add 零 before numbers, and I would ordinarily expect 三年九个月 to be appropriate. Maybe I should just chalk this up to poetic license usage.
Question: Why is there a 零 in 三年零九个月?