when is baby a month old??? 🤔🤔

sooo i know im not crazy lol.. my sister had her son on july 27th on august 24th, she posted saying he was one month old.. so im like buuut i thought he was born on the 27th?? then i was like no that's her son, im pretty sure she knows the day he was born, lol.. then i go look at my instagram and i posted the day he was born which was for sure the 27th... sooo august 24th was exactly 4 weeks since he was born.. soo do you go by weeks or would it be by the actual date????