I have been told that the following are the same:
It has been twenty years since I last saw her.
It is twenty years since I have seen her.
But I somehow do not understand why; usually the present perfect is somehow related to present ("I have not seen her recently"). I would tend to say that he has been seeing her since then (twenty years ago).
Also why does the first one contain "It has been" and the other one "It is". Does it matter?
I'd like to compare the present perfect usage with the simple past one.
First, the simple past.
All three sentences are asking the same thing, the speaker would like to know when did the two people last see each other. Specifically, he's asking for a point in time.
The respondent could reply like this:
The act of seeing the woman is a finished event, completed at a specific time. That episode cannot be repeated and we know exactly when this event occurred. This tense is usually referred to as the simple past.
Alternatively...
The speaker wants to know how much time has elapsed since their last meeting. In other words, the act of not seeing each other. The action of not seeing somebody is a continuous one, it began in the past and continues to the present. The two people have not seen each other since 1993 or if you prefer, for twenty years.
Therefore, the present perfect tense is the most acceptable one in this type of situation, and the most recommended (without any contractions) for formal writing.
In spoken English; however, it is common to use this type of construction
it is + period + since + past or perfect tense.
The meaning is the same as the present perfect sentences above.
http://www.grammaring.com/its-been-days-weeks-months-etc-since
No I dont think it matters. Both answers have the same meaning and in no way are they different but through sentence patterns. You can jumble up a bunch of words in a sentence and if it is grammatically correct you can have the same meaning or different. In your case, it is probably same.
In an isolated sentence, there is no difference between the simple and the perfect tenses. In a sequence of sentences, though, the simple tenses (especially the past) generally create the impression of sequence. "I went down the hall. I found the note. I burst into tears." We assume that those things happened in that order. A perfect tense allows you to make a side statement about a different time. "I went down the hall. I found the note. She had told me she was unhappy, but I hadn't listened. I burst into tears." The past perfect lets me refer to an event in the more remote past without changing the expected time of the next sentence.
The simple present in English isn't usually narrative, but it can be, and that's how it seems to be in your examples. "She walks down the sidewalk. Her hair flies in the wind. It has been twenty years since I last saw her. I wave my hand tentatively."
