using System;

using System.Collections.Generic;

​

namespace DateFormatTest

{

    public class Program

    {

        private const string TODAY_FORMAT = "HH:mm";

        private const string OTHER_DAY_FORMAT = "yyMMdd";

        private List<DataItem> _Data = new List<DataItem>

        {

            new DataItem { Id = 1, When = DateTime.Today.AddDays(-1) },

            new DataItem { Id = 2, When = DateTime.Now },

            new DataItem { Id = 3, When = DateTime.Today.AddDays(1) },

            new DataItem { Id = 4, When = null }

        };

        public void Main()

        {

            foreach(DataItem item in _Data)

            {

                Console.WriteLine("Id: " + Convert.ToString(item.Id) + ", When: " +FormatDate(item.When));

            }

        }

        // Helper method to format date depending on today or other day.

        private string FormatDate(DateTime? val)

        {

            return val.HasValue ? 

                ((DateTime)val).Date == DateTime.Today ? ((DateTime)val).ToString(TODAY_FORMAT) : ((DateTime)val).ToString(OTHER_DAY_FORMAT)

                : "n/a";

        }

    }

    public class DataItem

    {

        public int Id { get; set; }

        public DateTime? When { get; set; }

    }

}