■陈根花老师在“英语语法网”有一篇很好的文章，可以解决你的问题。

《that day, this day, these days, those days前用介词吗》

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■these days 的意思是“近来”“如今”“现在”，其前习惯上不加介词。如：

I rarely see her these days. 近来我很少见到她。

A pound doesn’t go far these days. 现今一英镑不经一用。

News travels quickly these days. 近来消息传播得很快。

Jobs are hard to come by these days. 近来，很难找到工作。

I don’t have much time to read these days. 我这些日子没什么时间读书。

■those days 前通常要用介词 in，短语 in those days 的意思是“那时”“当时”“那年头”“那年月”等，in those days 在许多情况下与 at that time 意思相同。如：

We were poor in those days. 那时我们很穷。

Life was hard in those days. 在那些日子里，生活很苦。

I was still living with my parents in those days. 我那时还跟父母住在一起。

Only a small number of people went abroad in those days. 那时只有少数人出国。

It wasn’t quite the done thing for women to drink in pubs in those days. 那年月女的到酒店喝酒可不成体统。

注意：these days 与 in those days 虽然形式相近，但其中的一个习惯上要用介词，而另一个则习惯上不用介词，注意不要搞混淆了。

■原文见：http://www.yygrammar.com/Article/201011/1809.html