Past simple v.s. Present Perfect

[Lost_Soul]

[Jamie (K)]

The present perfect tense indicates a relationship with the present. When someone says, "How long has it been since you've seen your son?" it indicates that he wants to know how much time has passed between the last time the person saw his son and now (meaning the time when the person is asking).

It's possible to say, "How long has it been since the last time you saw your son?" In that case [the last time you saw your son] is one noun phrase, with "you saw your son" being an adjective phrase modifying "time". It can be in the simple past, because it is confined to the time in the past when the person saw his son, not a span of time from then until now.

[Lost_Soul]

Hi, Jamie

I get your idea, thank you for your explanation.