强调句、定语从句还是状语从句

It was "at" seven o'clock "that" we reached the station.

请问此句是强调句，为什么又是定语从句? 书上说定语从句：从句“that we reached the staion”修饰的是it=the time. 那为什么不把从句直接写到先行词后 It that we reached the station was at seven o'clock . 希望老师帮我分析下，谢谢！

另外，为什么主句时间前没加介词 at 就成了when引导的时间状语从句了，如：It was seven o'clock when we reached the station.（状语从句）而加介词at就成为 It was "at" seven o'clock "that" we reached the station.(强调句）

那如果写成It was at seven o'clock when we reached the station.是否正确呢？这又是什么从句呢？