another story　
另一回事

I like to stay up late. It's the best time for me to write.
（我喜欢熬夜。那是我写作最好的时间。）
But it's quite another story for those of us who have to be at work by 8 a.m.
（但那对我们这些得在八点前上班的人来说就完全是另外一回事了。）