using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Period> periodList = PreparePeriods();

​

        // Get start dates

        List<DateTime> startDates = periodList

            .Where(p => p.StartDate.HasValue)

            .Select(p => p.StartDate.Value)

            .ToList();

        // Get end dates

        List<DateTime> endDates = periodList

            .Where(p => p.EndDate.HasValue)

            .Select(p => p.EndDate.Value)

            .ToList();

        // Clear list of periods

        periodList.Clear();

        // Add start dates which are bigger than LAST end date with NULL end date period

        startDates.Where(s => s > endDates.Max())

            .ToList()

            .ForEach(s => periodList.Add(new Period() { StartDate = s, EndDate = null }));

        // Add end dates which are smaller than FIRST start date with NULL start date period

        endDates.Where(e => e < startDates.Min())

            .ToList()

            .ForEach(e => periodList.Add(new Period() {StartDate = null, EndDate = e}));

        // Match other dates and add them to list

        startDates.Where(s => s < endDates.Max())

            .ToList()

            .ForEach(s => periodList.Add(new Period()

                                         {

                                             StartDate = s,

                                             EndDate = endDates.Where(e => e > s).Min()

                                         }));

        // Oder period list

        periodList = periodList.OrderBy(p => p.StartDate).ToList();

        PrintPeriods(periodList);

    }

    static List<Period> PreparePeriods()

    {

        int year = 2015;

        // Make list of periods ready

        List<Period> periodList = new List<Period>

        {

            new Period

            {

                StartDate = null,

                EndDate = new DateTime(year, 12, 1),

            },

                new Period

            {

                StartDate = new DateTime(year, 12, 2),

                EndDate = null,

            },

                new Period

            {

                StartDate = null,

                EndDate = new DateTime(year, 12, 4),

            },

                new Period

            {

                StartDate = new DateTime(year, 12, 6),

                EndDate = new DateTime(year, 12, 8),

            },

                new Period

            {

                StartDate = new DateTime(year, 12, 9),

                EndDate = null,

            },

                new Period

            {

                StartDate = null,

                EndDate = new DateTime(year, 12, 10),

            },

                new Period

            {

                StartDate = new DateTime(year, 12, 11),

                EndDate = null,

            }

        };

        return periodList;

    }

    static void PrintPeriods(List<Period> periodList)

    {

        periodList.ForEach(p =>

                           Console.WriteLine(string.Format("{0} | {1}",

                                                           p.StartDate,

                                                           p.EndDate)));

    }

}

​

public class Period

{

    public DateTime? StartDate { get; set; }

    public DateTime? EndDate { get; set; }

}