using System;

public class Program

{

    public static void Main()

    {

        //Leading zeros vs leading spaces       

        DateTime exampleDT = new DateTime(2018, 8, 1, 0, 0, 0, 0);

        string exampleHHMMSSusingD2  = string.Format(exampleDT.Hour.ToString("D2")) + ":" + string.Format("{0,3}", exampleDT.Minute.ToString("D2")) + ":" + string.Format("{0:D3}", exampleDT.Second.ToString("D1") +" 2 leading zeros");

        string exampleHHMMSSusing000 = string.Format("{0:00}", exampleDT.Hour.ToString()) + ":" + string.Format("{0:D2}", exampleDT.Minute.ToString()) + ":" + string.Format("{0:11}", exampleDT.Second.ToString()+" no 2 leading zeros");

        Console.WriteLine(exampleHHMMSSusingD2);

        Console.WriteLine(exampleHHMMSSusing000);

        Console.WriteLine();

        //formatting a string of zeros

        Console.WriteLine(string.Format("{0:00} string", "0000")); 

        Console.WriteLine(string.Format("{0:00} string", Convert.ToInt32("00000")));

        Console.WriteLine(string.Format("{0:00} string", Convert.ToInt32("12345"))); 

        Console.WriteLine(string.Format("{0:00} string", Convert.ToInt32(null))); 

        Console.WriteLine();

​

        //using PadLeft

        Console.WriteLine(string.Format("{0:00} pad", "1111".PadLeft(2, '0')));

        Console.WriteLine(string.Format("{0:00} pad", "11".PadLeft(2, '0')));

        Console.WriteLine(string.Format("{0:00} pad", "1".PadLeft(2, '0')));

        Console.WriteLine(string.Format("{0:00} pad", "".PadLeft(2, '0')));

    }

}