Podemos decir:
- It is three years since I (last) saw Bill, o bien
It is three years since I have seen Bill.
I last saw Bill three years ago, o bien
I haven’t seen Bill for three years.
– It is two months since Tom (last) smoked a cigarette, o bien
It is two months since Tom has smoked a cigarette.
He last smoked a cigarette two months ago, o bien
He hasn’t smoked a cigarette for two months.
Podemos usar la construcción it is … since sin el adverbio «last»:
– It is two years since he left the country.
Esto, sin embargo, solo es reemplazable por:
– He left the country two years ago.
No podríamos emplear un pretérito perfecto negativo aquí como en la oración acerca de Bill arriba. «He hasn’t been (living) in this country for the last two years» es posible pero no es un equivalente exacto de «he left two years ago.»
La construcción puede usarse en el pasado:
– He invited me to go riding with him. But it was two years since I had ridden a horse. (I hadn’t ridden a horse for two years previous to the invitation so I wasn’t sure that I would enjoy it.)
A Practical English Grammar (A.J. Thomson & A.V. Martinet) 