"tanjyoubi wa itsu desu ka" or "tanjyoubi itsu desu ka" The person you're referring to is implied so you don't need "anata no" it may come off as unnatural. You can, of course still use it. (sorry, I dont have kana on this computer.)

それはよくないです。 『誕生日いつですか』とはいいです。

When is your birthday? = あなたのたんじょうびはいつですか。 My birthday is January 16th. = わたしのたんじょうびはいちがつじゅうろくにちです。 January = いちがつ 16 (of the month) = じゅうろくにち *We have "Japanese counter" for each things. I hope this was helpful.