Should I be annoyed?

Me and my fiancé have been together nearly two years now. For some reason he can't remember my birthday, it's really frustrating because I celebrated my 21st this year (march) and three times since then whenever he is filling in a form on ny behalf or doing something with insurance he seems to think my birthday is in October! I only found out the second time he got it wrong was because his ex's birthday was in October, I found myself getting really wound up because I felt like how can he want to marry me if he keeps thinking my birthday is his ex's birthday! He even had the cheek to try and argue the fact, like I don't know when my own birthday is!! I know you ladies think it's probably nothing but what would you do?? He is never normally one to forgets things as well!