how to convert string to date

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C#

ASP.NET

here i have to store b'day in table.i've took 3 dropdownlist for day.month,year respectively.
and stors it's value to a hiddenfield.
bt it shows me the error that."

String was not recognized as a valid DateTime.

"

here i gave u chunk of code of what i did..
if there is any mistake then plz point me out...

protected void Button1_Click(object sender, System.EventArgs e)
        {
HiddenField1.Value = ddl_mnth.SelectedValue + ddl_day.SelectedValue + ddl_yr.SelectedValue;
          
                cn.Open(); 
                DateTime dt;
              
                string role = "user";
                dt = Convert.ToDateTime(HiddenField1 .Value );
                string cstr = "INSERT INTO register VALUES('" + unm_txt.Text + "','" + fnm_txt.Text + "','" + lnm_txt.Text + "','" + eml_txt.Text + "','" + ddl_lv.SelectedItem.Value + "','" + pwd_txt.Text + "','"+ gd.SelectedValue  +"','" + HiddenField1.Value  + "','" + phn_txt.Text + "')";
                SqlCommand cmd = new SqlCommand(cstr, cn);
                cmd.ExecuteNonQuery();
cn.close();
}