how to input date

I am designing a web form in that enable user to enter information for registration.
I cant decide what strategy I should use to input date for date of birth. there is a calendar control available but it too slow and consume larger space in webform.
then I decided to use three DropDownList boxes to selcet date, month and year individually. but how to store selected date in DateTime instance?
I use this
DateTime dob; dob.Date = (DateTime)(ddlmonth.SelectedItem.Text + ddlday.SelectedItem.Text + "," + ddlyear.SelectedItem.Text);

but this is producing error.
I think this strategy is not good.
isn't there any control in most featured ASP.Net technology that solve my problem?

...