Curious structure with 的 in conversation
The link you attached has well explained 是....的 structure. 

Basically, 是......的 structure connotes past tense when used in the interrogative sentence. 

Say 你什么时候去？ vs 你是什么时候去的: the former means when will you go? The latter means when did you go? or when have you been (there)? 

A side note: sometimes 是 can be omitted in colloquial contexts. For example,  你什么时候去的， which is the same as 你是什么时候去的. 

Hope this could make it more clear to you.