The structure It's been + time + since is one of the most natural and common ways to express how much time has passed since something last happened. It measures the gap between a past event and the present moment, and it carries a emotional weight — often a sense of nostalgia, surprise, or emphasis on how long the gap has been.
Think of it this way: you are standing at the present moment and looking back across time towards the last occasion something happened. The structure lets you announce the length of that gap in a single, fluid sentence:
The structure has several natural variations you will hear in everyday English, all conveying the same meaning:
The word last is very commonly added before the verb in the since clause. It emphasises that you are talking about the most recent occasion — the last time something happened before now. It is optional but makes the sentence feel more natural and conversational:
This is a critical rule. After since in this structure, the verb is always in the Past Simple — never the Present Perfect, never the Present Simple. The since clause is describing a specific past event — the last time something happened — which is why the Past Simple is correct.
✗ It's been since three years I saw her. · It's since months we spoke.
✓ It's been three years since I saw her. · It's been months since we spoke.
The fixed order is: It's been + time expression + since + clause. The time expression always comes between been and since — never before It's been and never after since. The word since introduces the clause describing the past event; it does not introduce the time expression.
✗ It's been three years since I see her. · It's been ages since we have spoken. · How long since you are here?
✓ It's been three years since I saw her. · It's been ages since we spoke. · How long has it been since you were last here?
The since clause in this structure describes the last occasion something happened — a specific completed past event. Completed past events use the Past Simple. The Present Simple sounds like a permanent fact; the Present Perfect cannot be used in a since clause in this structure. Always use the Past Simple after since here.
✗ It is three years since I saw her. · It is ages since we spoke.
✓ It has been three years since I saw her. · It's been ages since we spoke.
The structure requires the Present Perfect of be: has been (or the contracted form 's been). Using is (Present Simple) is incorrect in standard modern English for this structure. The Present Perfect is essential because you are measuring a gap that stretches from a past point up to now — exactly the territory of the Present Perfect.
✗ It's been three years since I don't see her. · It's been ages that we haven't spoken.
✓ It's been three years since I last saw her. (It's been + time + since + Past Simple)
✓ I haven't seen her for three years. (Subject + haven't/hasn't + PP + for + time)
These two structures express the same meaning but are built differently. It's been + time + since uses a positive Past Simple in the since clause — it talks about the last occasion it happened, not about the gap of non-happening. I haven't seen her for three years uses the negative Present Perfect. Both are correct, but you cannot mix the two: the since clause must have a positive Past Simple verb, not a negative form.