It's or It's been

2) is correct...

Both are correct.

why do you think 1) is correct?

Because "It is [period of time] since [something happened]" is a common and valid way of discussing an elapsed period of time in English.

One of many sources:

A practical English grammar (4th edition) by A.J Thomson & A.V Martinet, page 171:



You can use all the tenses of 'to be' to express varying ideas connected with this.

Last Tuesday it was ten years since Scotland won a game against England.

Today it is ten years since Scotland won a game against England.

Next Tuesday it will be ten years since Scotland won a game against England.

i suppose it's technically correct grammar, but i think "it's been" (it has been) sounds much better...

We weren't asked which one 'sounds better'. We were asked which one was correct. Grammar books say both are correct. Both 'sound' equally good to me.