Need help understanding

B.A

Ladies, if my cycle was 29 days December 2018 ( i spotted days before)

Glow is saying 29 days December 2019.

Is this accurate? Im usually 26 days which would mean im late... but according to the app im getting period tomorrow.

Question is, how accurAte is the app??? If i was 29 cycle days in Dec 2018. Is that accurate that i will be 29 days Dec 2019?