it be +序数词+time.+that+完成时,这个句型中的it可以是哪些词来做主语?

It is the first time that ..这句叫无主句,就是没法确定主语的句子,但是一个句子必须要有主语,所以就拉了个it 来顶.还有下列句子都是：
It's time to go now.
It's five o'clock now.
It will be fine tomorrow.
It's two miles away.