I suppose that the sentence:
I've won the lottery. This is the second time this happened.
isn't correct - it must be:
I've won the lottery. This is the second time this has happened.
But what is the reason for that?
Is it because the present tense in the beginning "This is the second time" determines the present perfect tense in the ending "this has happened"?
Or is it because we just have to use the present perfect in this context?