By the time you arrive in London,we will have stayed there for two weeks.求分析,时态,用法,

By the time you arrive in London,we will have stayed there for two weeks.求分析,时态,用法,
还有定语从句when ,被省略了么?为什么用will 然后又用了have done 的句式?
句子的意思是不是：当你到达伦敦的时候，我们已经在那边两个星期了？