语法分析,下句中的which是什么用法

题目

语法分析,下句中的which是什么用法
He came back late ,by which time all the guests had already left .
which怎么能修饰time呢,它不是个连词吗

答案

which指代前面一句的内容,是定语从句中的关系代词.
介词+关系代词=关系副词
所以by which time=when
和这句He came back late ,when all the guests had already left .
一样的意思.