as soon as he entered the door，as soon as 不是引导时间状语从句吗？为什么后面能跟过去时呢？

They said that they would ring me up as soon as they received the letter. They will ring me up as soon as they receiv the letter .是可以用过去时态的．

为什么不能跟过去时？这句话的意思本来就是指过去发生的事情。要么过去时要么完成时，现在时和将来时可能么?