it引导分裂句

在高中英语北师大版教程上发现这个句子：

It is December that we are going away.

教程讲解的是it 引导分裂句可以把新信息和已知的信息联系起来，it在这里指的是时间，那that在这里是什么从句呢？

It is December that we are going away.= It is December that we are going away.

【说明】 It is ... that 是一种强调结构，这里被强调的成分是时间状语  December。