Leading Zero Date Format C#

I have this function...

private string dateConvert(string datDate)
{
        System.Globalization.CultureInfo cultEnGb = new System.Globalization.CultureInfo("en-GB");
        System.Globalization.CultureInfo cultEnUs = new System.Globalization.CultureInfo("en-US");

        DateTime dtGb = Convert.ToDateTime(datDate, cultEnGb.DateTimeFormat);
        datDate = dtGb.ToString(cultEnUs.DateTimeFormat.ShortDatePattern);

        return datDate;
}

But I want it with the leading zero still on lower digits (1-9) so the date is 11-09-2009 (mm-dd-yyyy)...

Now If I wasn't converting it id use string.Format("{0:d}", dateVar) how do I do this in the conversion?

***** Solution *****

Used a slightly modified version of the answer below (i.e. one that would render).

Convert.ToDateTime(datDate).ToString("MM-dd-yyyy");