date time format in c#?

DateTime.Now.ToString("yyyy-MM-dd hh:mm:ss.fff")

Console.WriteLine(DateTime.Now.ToString("yyyy-MM-dd hh:mm:ss:fff"));

public static string getDate(string strDate, string strFormat)
    {
        if (clsCommon.IsDate(strDate))
        {
            if (strFormat == "")
                return String.Format("{0:yyyy-MM-dd H:mm:ss zzz}", Convert.ToDateTime(strDate));
            else
                return String.Format(strFormat, Convert.ToDateTime(strDate));
        }
        else
        {
            return strDate;
        }
    }


getDate(strdate,"");

string time = String.Format(DateTime.Now.ToString(), "dd/MM/yyyy HH:mm:ss.fff");

private void Button1_Click(object sender, EventArgs e)
        {
         DateTime d = new DateTime();
         MessageBox.Show(d.Year + "-" + d.Month + "-" + d.Date + d.TimeOfDay);
        }

 private void frmReturnPurchase_Load(object sender, EventArgs e)
     
  { 
DtpRethurnBillDate.Format = DateTimePickerFormat.Custom;
          DtpRethurnBillDate.CustomFormat = "dd/MM/yyyy"

}