Present Perfect is a Present Tense

Present Perfect is a really confusing tense, on one hand there is not doubt that it belongs to the family of the present tenses, on the other it may sometimes refer to the past:

I have found my cat! It was on the tree.

So, what tense is it, after all? As I said earlier it is a present tense and the most important implication of this is we don't have to worry about while using the reported speech:

"We won't come to your party".

They've said they won't come to my party.

As you can see, "won't" stays the same because there no backshift of tenses in the present tenses. If we took away the "'ve" from the sentence, the things would look different:

"We won't come to your party".

They've said they wouldn't come to my party.