when my daughter was born in counted the days but now I'm on my 3rd baby if he happens to be born on the 4th I'm going to say he will be 1 month on the following 4th no matter the days lol.
I think the baby would be one month on the day of the month they were born... like born on March 3rd they would be 1 month on April 3rd.
say she is 1 month on the 1st