Dæmi

using System;

public class Program
{
    public static void Main()
    {
        int targetYear = -34567;
        int year = 2011;
        int month = 1;
        int date = 1;
        int day = 6;
        string f13s = "";

        /*
            Day:
            0 = Sunnudagur
            1 = Mánudagur
            2 = Þriðjudagur
            3 = Miðvikudagur
            4 = Fimmtudagur
            5 = Föstudagur
            6 = Laugardagur
        */

        while (year >= targetYear)
        {
            date--;
            day--;

            if (day == -1) day = 6;

            if (date == 0)
            {
                month--;

                if (month == 0)
                {
                    month = 12;
                    year--;
                }

                date = DaysInMonth(year, month);
            }

            if (year == targetYear && date == 13 && day == 5) f13s = month.ToString() + f13s;
        }

        Console.WriteLine(f13s);
    }

    public static int DaysInMonth(int year, int month)
    {
        switch (month)
        {
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12: return 31;
            case 2: return IsLeapYear(year) ? 29 : 28;
            default: return 30;
        }
    }

    public static bool IsLeapYear(int year)
    {
        if (year % 400 == 0) return true;
        else if (year % 100 == 0) return false;
        else if (year % 4 == 0) return true;
        else return false;
    }
}