Setting AR attribute from date_select values?

Rails
1

I have a date select:

<%= form_fields.date_select :birthday_on, :order => [:month, :day,
:year], :start_year => 1900 %>

In my controller I want to set the individual attribute. How do I do
this?

user.birthday_on = params[:user][:birthday_on]

How do I convert this to a date so it set properly?

Thanks for your time!