It is until 12 o'clock that i finished my homework

问题描述:

It is until 12 o'clock that i finished my homework
为什么是that 而不是when 或/

是强调句,意思是直到12点我才完成了我的作业.
而把until去掉,再把that换成when的话意思是当我做完作业时是12点,而这个句子不是定语从句,不能用/