DouKaku?

#!/usr/bin/env ruby
require 'date'

from = DateTime.now
to = DateTime.parse("2013-12-31")

friday = (from..to).inject(0) do |friday, date|
    if date.mday == 13 and date.wday == 5 then
        puts date.strftime('%Y-%m-%d')
        friday + 1
    else
        friday
    end
end

puts friday

#include <ctime>
#include <iostream>

int dayOfWeek(int y, int m, int d)
{
    if((m == 1) || (m == 2))
    {
        y--;
        m += 12;
    }
    return (y + y / 4 - y / 100 + y / 400 + (13 * m + 8) / 5 + d) % 7;
}

int main(int, char* [])
{
    std::time_t today   = std::time(0);
    std::tm*    todayTm = std::localtime(&today);

    int count = 0;
    int m     = (todayTm->tm_mday <= 13) ? todayTm->tm_mon : todayTm->tm_mon + 1;
    for(int y = todayTm->tm_year + 1900; y <= 2013; ++y)
    {
        while(m <= 12)
        {
            if(dayOfWeek(y, m ,13) == 5)
            {
                std::cout << y << "/" << m << "/13" << std::endl;
                ++count;
            }
            ++m;
        }
        m = 1;
    }
    std::cout << "total: " << count << std::endl;

    return 0;
}