前一博文中我借回答老师的提问，解释了语块back in...现象和用法。本博我们不妨再多见识两个类似表述：

back in the day

used for talking about a time in the past, usually when you are remembering nice things about that time

Back in the day, we had an apartment with a swimming pool.

My dad's always talking about how everything was back in the day.

现在明了了：back in the day有点像个idiom，是回顾过去的好时光，并非在说“白天”我们所做的事情。

back in the days

at a particular time in the past

I was a fan back in the days when the band wasn't yet famous.

前博链接：

 http://blog.sina.com.cn/s/blog_3cb5ffee0102wale.html