Checking DOB -not entering Future Date

Hi,
   In RegistrationPage I include DOB option.For this,I use Months in Dropdownlist and geting Date,Year in seperate Textboxes.It will be stored in DB successfully.But the problem is when I give Future Date as DOB it'll saved in DB.Actually this's wrong.So,I have to check Dropdownlist,Textboxes for not entering FutureDate.Can anyone help me to solve this?