这不是主语从句,而是强调句,强调时间状语,故that不能省略
在强调句中,把It is … that去掉之后,还基本上像个句子.
如去掉It is … that之后the second time I have been to Beijing.还勉强是个句子.
在主语从句中,把It is … that去掉之后,就已经完全不像一个句子了.
如把主语从句It is strange that he have so many friends.中的It is … that去掉之后,strange he have so many friends就不是句子了