句型是：It’s +一段时间+since+过去时(或It has been +一段时间+since+过去时)
1.It is ten years since she left me.她离开我已经10年了.
2.It’s a long time since our last vacation.从上个假期到现在已经很久了.
3.How long is it since we visited your mother?自从我们看望你母亲以来到现在有多长时间了?
所以也可以说：it has been 100 years since Qinghua university was found.