Since and the perfect tense

I know that saying:

(A) I am here since six o’clock. or (B) I was here since six o’clock.

would sound unidiomatic because of using "since" and its requisite tense which is present perfect. For example:

(C) - I have been here since six o’clock.

But I came across another sentence somewhere in a textbook which caused me to doubt. Is it possible to say:

(D) - I had been there since six o’clock.

If yes, what's the semantic difference between "C" and "D", and when should I use each one?

Answer

Your (D) is past perfect ... it refers to a time in the past just as (C) refers to a time in the present.

So (D) means that you were “there” from six o’clock to whatever past time you are talking about.

When Amy finally arrived at the bar at eight o’clock I had been there since six o’clock.