DateTime expectedDate;
if (!DateTime.TryParse("07/27/2012", out expectedDate))
{
    Console.Write("Luke I am not your datetime.... NOOO!!!!!!!!!!!!!!");
}

string[] formats = { "MM/dd/yyyy", "M/d/yyyy", "M/dd/yyyy", "MM/d/yyyy" };
DateTime expectedDate;
if (!DateTime.TryParseExact("07/27/2012", formats, new CultureInfo("en-US"),
                            DateTimeStyles.None, out expectedDate))
{
    Console.Write("Thank you Mario, but the DateTime is in another format.");
}

private bool IsValidDateFormat(string dateFormat)
    {
        try
        {
            String dts=DateTime.Now.ToString(dateFormat);
            DateTime.ParseExact(dts, dateFormat, CultureInfo.InvariantCulture);
            return true;
        }
        catch (Exception)
        {
            return false;
        }
    }

DateTime.ParseExact("qq", "qq", null) == DateTime.Today
DateTime.ParseExact("myy", "501", null) == "05/01/2001"

d,D,f,F,g,G,m,M,o,O,r,R,s,T,u,U,y,Y

try
{
  String formattedDate = DateTime.Now.ToString(dateFormat);
  DateTime.Parse(formattedDate);
  return true;
}
catch (Exception)
{
  return false;
}