function calendar(date) {
  // If no parameter is passed use the current date.
  if(date == null)
     date = new Date();

  day = date.getDate();
  month = date.getMonth();
  year = date.getFullYear();

  months = new Array('Styczeń','Luty','Marzec','Kwiecień','Maj','Czerwiec','Lipiec','Sierpień','Wrzesień','Październik','Listopad','Grudzień');

  this_month = new Date(year, month, 1);
  next_month = new Date(year, month + 1, 1);

  // Find out when this month starts and ends.
  first_week_day = this_month.getDay();
  days_in_this_month = Math.round((next_month.getTime() - this_month.getTime()) / (1000 * 60 * 60 * 24));

  calendar_html = '<table id="calendar_table" border="0" width="100%">';
  calendar_html += '<tr><td colspan="7" style="text-align: center; padding-bottom:15px;">' + months[month] + ' ' + year + '</td></tr>';
  calendar_html += '<tr id="headers"><th>Pn</th><th>Wt</th><th>Śr</th><th>Cz</th><th>Pt</th><th>So</th><th>Nd</th></tr>';
	calendar_html += '<tr>';

  // Fill the first week of the month with the appropriate number of blanks.
  for(week_day = 0; week_day < first_week_day - 1; week_day++) {
    calendar_html += '<td> </td>';
  }

  week_day = first_week_day - 1;
  for(day_counter = 1; day_counter <= days_in_this_month; day_counter++) {
    week_day %= 7;
    if(week_day == 0)
      calendar_html += '</tr><tr>';

    // Do something different for the current day.
    if(day == day_counter)
      calendar_html += '<td style="text-align: center;"><b>' + day_counter + '</b></td>';
    else
      calendar_html += '<td style="text-align: center;"> ' + day_counter + ' </td>';

    week_day++;
  }

  calendar_html += '</tr>';
  calendar_html += '</table>';

  // Display the calendar.
  document.write(calendar_html);
}