Date Validation at code behind in c#

        string strDOB = ""; string strDebtType = "";
        txtJudgementdate.Text = hdnJudgementdate.Value;
        txtCostCertificationDate.Text = hdnCostCertificationDate.Value;
        txtDefaultDate.Text = hdnDefaultDate.Value;
        if (ddlDay.SelectedIndex > 0)
            if (ddlMonth.SelectedIndex > 0 && ddlYear.SelectedIndex > 0)
            {
                if (ddlMonth.SelectedIndex == 4 || ddlMonth.SelectedIndex == 6 || ddlMonth.SelectedIndex == 9 || ddlMonth.SelectedIndex == 11)
                {
                    if (ddlDay.SelectedIndex > 30)
                    {
                        lblError.Text = "Please Select Valid Date of Birth.";
                        return;
                    }
                }
                else if (ddlMonth.SelectedIndex == 2)
                {
                    if (Convert.ToInt32(ddlYear.SelectedValue) % 4 == 0)
                    {
                        if (ddlDay.SelectedIndex > 29)
                        {
                            lblError.Text = "Please Select Valid Date of Birth.";
                            return;
                        }
                    }
                    else
                    {
                        if (ddlDay.SelectedIndex > 28)
                        {
                            lblError.Text = "Please Select Valid Date of Birth.";
                            return;
                        }
                    }
                }
               
                strDOB = new DateTime(Convert.ToInt32(ddlYear.SelectedValue), Convert.ToInt16(ddlMonth.SelectedIndex), Convert.ToInt16(ddlDay.SelectedValue)).ToString("dd/MM/yyyy");
                //strDOB = DateTime.Parse(ddlDay.SelectedValue + "/" + Convert.ToInt16(ddlMonth.SelectedIndex) + "/" + Convert.ToInt32(ddlYear.SelectedValue)).ToString("dd/MM/yyyy");
            }
            else
            {
                lblError.Text = "Please Select Valid Date of Birth.";
                return;
            }
        else if (ddlMonth.SelectedIndex > 0)
            if (ddlDay.SelectedIndex > 0 && ddlYear.SelectedIndex > 0)
            {
                if (ddlMonth.SelectedIndex == 4 || ddlMonth.SelectedIndex == 6 || ddlMonth.SelectedIndex == 9 || ddlMonth.SelectedIndex == 11)
                {
                    if (ddlDay.SelectedIndex > 30)
                    {
                        lblError.Text = "Please Select Valid Date of Birth.";
                        return;
                    }}
                else if (ddlMonth.SelectedIndex == 2)
                {
                    if (Convert.ToInt32(ddlYear.SelectedValue) % 4 == 0)
                    {
                        if (ddlDay.SelectedIndex > 29)
                        {
                            lblError.Text = "Please Select Valid Date of Birth.";
                            return;
                        }
                    }
                    else
                    {
                        if (ddlDay.SelectedIndex > 28)
                        {
                            lblError.Text = "Please Select Valid Date of Birth.";
                            return;
                        }
                    }
                }
                strDOB = new DateTime(Convert.ToInt32(ddlYear.SelectedValue), Convert.ToInt16(ddlMonth.SelectedIndex), Convert.ToInt16(ddlDay.SelectedValue)).ToString("dd/MM/yyyy");
            }
            else
            {
                lblError.Text = "Please Select Valid Date of Birth.";
                return;
            }
        else if (ddlYear.SelectedIndex > 0)
            if (ddlMonth.SelectedIndex > 0 && ddlDay.SelectedIndex > 0)
            {
                if (ddlMonth.SelectedIndex == 4 || ddlMonth.SelectedIndex == 6 || ddlMonth.SelectedIndex == 9 || ddlMonth.SelectedIndex == 11)
                {
                    if (ddlDay.SelectedIndex > 30)
                    {
                        lblError.Text = "Please Select Valid Date of Birth.";
                        return;
                    }}
                else if (ddlMonth.SelectedIndex == 2)
                {
                    if (Convert.ToInt32(ddlYear.SelectedValue) % 4 == 0)
                    {
                        if (ddlDay.SelectedIndex > 29)
                        {
                            lblError.Text = "Please Select Valid Date of Birth.";
                            return;
                        }
                    }
                    else
                    {
                        if (ddlDay.SelectedIndex > 28)
                        {
                            lblError.Text = "Please Select Valid Date of Birth.";
                            return;
                        }
                    }
                }
                strDOB = new DateTime(Convert.ToInt32(ddlYear.SelectedValue), Convert.ToInt16(ddlMonth.SelectedIndex), Convert.ToInt16(ddlDay.SelectedValue)).ToString("dd/MM/yyyy");
            }
            else
            {
                lblError.Text = "Please Select Valid Date of Birth.";
                return;
            }