It is the first time I have read the book.
It is the first time I have seen the movie.
It is the first time I have met the Smiths.
It is the first time I have listened to this piece of msic.
It is the first time they have been to America.
It is the first time 后面接的是一个定语从句，修饰前面的first time。这个定语从句之所以用现在完成时态，是因为主句It is the first time 用的是一般现在时态，定语从句使用现在完成时态是说明“XXX从过去算到现在为止是第一次做XXX”（当然也可以有“第二次”、“第三次”......）的意思。也可以说“This is the first time ...”。定语从句前的连接词是that，因为time前还有序数词first（second、third....）修饰，但that一般省略。
如果把It/This is the first time ... 中的is改为过去时was，this要改that，后面的定语从句就要相应地用过去完成时态。例如：
That was the first time I had taken a train to travel.那是我第一次乘火车旅行。
That was the first time I had left Guilin. 那是我第一次离开桂林。
For many of them, it was the first time they'd ever voted. 对于其中的许多人来说，这是他们一生中第一次投票。
It was the second time that she had visited London. 那是她第二次游览伦敦。