Birthday input for registration form question

[eluser]Kanra[/eluser]
I'm writting a registration form and need to get user's date of birth.

I'm having three combobox for "day,month and year"

I want to populate the "day" combobox according to selected value of month.

Do I have to use javascript to fulfill this?

Does CI got some Class or Helper to help me?

All suggestions/advices are welcome

Thanks