using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var startDate = new DateTime(2019, 12, 28);

        var endDate = new DateTime(2020, 1, 3);         

        var listOfDays = CalculateRange(startDate, endDate);

        PrintList(listOfDays);

        startDate = new DateTime(2019, 12, 28);

        endDate = new DateTime(2019, 12, 31);           

        listOfDays = CalculateRange(startDate, endDate);

        PrintList(listOfDays);

    }

​

    public static IList<int> CalculateRange(DateTime startDate, DateTime endDate)

    {

        const int YEAR_FACTOR = 10_000;

        var listOfDays = new List<int>();

        var fromYear = startDate.Year;

        var fromDay = startDate.DayOfYear;

        var toYear = endDate.Year;

        var toDay = endDate.DayOfYear;

        var fromDayYear = fromDay * YEAR_FACTOR + fromYear;

        var toDayYear = toDay * YEAR_FACTOR + toYear;

        listOfDays.Add(fromDayYear);

        var limit = toDay;

        if (fromYear != toYear)

        {

            Console.WriteLine("- no -", limit);

            limit = new DateTime(toYear, 1, 1).AddDays(-1).DayOfYear;

        }

        Console.WriteLine("- {0} -", limit);

​

        for (var startDay = fromDay+1 ; startDay <= limit; startDay++)

        {

            listOfDays.Add(startDay * YEAR_FACTOR + fromYear);

        }

​

        if (fromYear != toYear)

        {

            limit = toDay;

            for (var startDay = 1; startDay < limit; startDay++)

            {

                listOfDays.Add(startDay * YEAR_FACTOR + fromYear);

            }

        }

​

        if (fromDayYear != toDayYear)

        {

            listOfDays.Add(toDayYear);

        }

​

        return listOfDays;

    }

​

    public static void PrintList(IList<int> list)

    {

        Console.WriteLine("Printing list");

        foreach (var item in list)

        {

            Console.WriteLine("{0:D7}", item);

        }

    }

}