It is a week since he has been to BeiJing.

问题描述:

It is a week since he has been to BeiJing.

不对
应该是It is a week since he went to Beijing.
或者是It has been a week since he went to Beijing为什么?Since 后面不是可以跟现在完成时吗?since引导的从句中从句用一般过去时主句用现在完成时