Dates in ExpressionEngine aren't exactly straightforward as you will know if you have ever worked with them. Regularly I get asked to build either a SafeCracker or Zoo Visitor form that will include a Date of Birth field. This is how I deal with that particular field.
You can find original article that this download belongs to at the following URL
http://johnhenry.ie/articles/managing-a-date-of-birth-field-in-an-expressionengine-form