He is the father

A man had sex with a woman and after 3 months she came with a baby and told him that he was the father. The man told her that was impossible, but the woman told him let's think about that
The woman: When did you sleep with me?
The man: 3 months age
The woman: When did I sleep with you?
The man: 3 months age
The woman: When did we sleep with each other?
The man: 3 months age
The woman: So 3 months + 3 months + 3 months = 9 months, so he is your son.