using System;

public class Program

{

    static void Main(string[] args)

    {

        var daysToCheck = 1000;

        var showWrongDatesCount = 3;

        var dateToCheck = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day);

        var wrongs = 0;

        for (int i = 1; i <= daysToCheck; i++) {

            var checkEndOfYear = new DateTime(dateToCheck.Year, 12, 31);

            var checkTotalDays = (int)checkEndOfYear.Subtract(dateToCheck).TotalDays;

            bool checkIsHighYear = (dateToCheck.Year - 2000) % 4 == 0;

            var checkDaysUntilEndOfYear = DaysUntilEndOfYear(dateToCheck.Day, dateToCheck.Month, checkIsHighYear);

            if (checkTotalDays != checkDaysUntilEndOfYear) {

                Console.WriteLine($"Number of days from {dateToCheck:d} till end of {dateToCheck.Year} is {checkTotalDays} but counted {checkDaysUntilEndOfYear}");

                wrongs++;

            }

            dateToCheck = dateToCheck.AddDays(-1);

            if (wrongs >= showWrongDatesCount) {

                break;

            }

        }

        if (wrongs == 0) {

            Console.WriteLine($"All {daysToCheck} days checked out fine!");

        }

    }

​

    static int DaysUntilEndOfYear(int day, int month, bool isHighYear)

    {

        //Implement this without using DateTime

        throw new NotImplementedException();

    }

}

​

​