How can I use "datetime" method?

public void ShowLongDate(string inputDate)
   {
   DateTime date;
   if (!DateTime.TryParse(inputDate, out date))
       {
       Console.WriteLine("Invalid date: " + inputDate);
       }
   else
       {
       Console.WriteLine(date.ToString("D"));
       }
   }

DateTime convertedDate = DateTime.Parse("17/12/2014");

DateTime convertedDate;

if (DateTime.TryParse("17/12/2014", CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal, out convertedDate))
{
    textBox1.Text = convertedDate.ToString(dateOutputFormat);
}
else
{
    textBox1.Text = "conversion fail";
}

CultureInfo provider = CultureInfo.InvariantCulture;

string dateInputFormat = "dd/mm/yyyy";
string dateOutputFormat = "dd MMMM yyyy";

private string stringToDate(string inputDate)
{
    DateTime convertedDate;

    if (DateTime.TryParseExact(inputDate, dateInputFormat, provider, DateTimeStyles.AssumeUniversal, out convertedDate))
    {
        return convertedDate.ToString(dateOutputFormat);
    }
    else
    {
       return "Invalid date conversion: " + inputDate;
    }
}

// test using a button-click
private void TestStringToDateConversion_Click(object sender, EventArgs e)
{
    textBox1.Text = stringToDate("17/12/2014");
}