How come Adeline shares the same birthday as her father?

Adeline's father offers to share his birthday with her when he realizes that she is ten..... she is thrilled by his offer.

“Ten years old! How time flies!” He looked into space and was lost in reverie. After a while he continued, “but we have to complete these landing cards! Tell you what. Why don’t I give you my birthday? Would you like that?”
“Yes, please, Father!” How wonderful! To share the same birthday as my father! I was thrilled!