Check this out:-
Create a calender and textbox in your design area.
And do this cs file:-
protected void Calendar1_SelectionChanged(object sender, EventArgs e)
{
TextBox1.Text = Convert.ToString(Calendar1.SelectedDate.ToShortDateString());
}
On button_click simply save the textbox value in your sql database
Also in your sqlserver take the date_column field datatype as "date".
Happy coding ;)