The tense of 経つ

I read in a novel this sentence below:
島に引っ越して一年が経つが、母親と俺が京都弁が抜けない。
Why 経つ? Should it not be たったbecause it has already happened.
I know present perfect should be translated as ている in most cases.
I have lived here for three years.
もうここに三年住んでいる。
But this 経つ one just doesn’t align with the rule.