function dateChanged(calendar) {
    // Beware that this function is called even if the end-user only
    // changed the month/year.  In order to determine if a date was
    // clicked you can use the dateClicked property of the calendar:
    if (calendar.dateClicked) {
      // OK, a date was clicked, redirect to /yyyy/mm/dd/index.php
      var y = calendar.date.getFullYear();
      var m = calendar.date.getMonth();     // integer, 0..11
      var d = calendar.date.getDate();      // integer, 1..31
      // redirect...
      window.location = "/" + y + "/" + m + "/" + d + "/index.php";
    }
  };

  function ourDateStatusFunc(date, y, m, d) {
    if (dateIsSpecial(y, m, d))
      return "special";
    else
      return false; // other dates are enabled
      // return true if you want to disable other dates
  };
    function catcalc(cal) {
        var date = cal.date;
        var time = date.getTime()
        // use the _other_ field
        var field = document.getElementById("f_calcdate");
        if (field == cal.params.inputField) {
            field = document.getElementById("f_date_a");
            time -= Date.WEEK; // substract one week
        } else {
            time += Date.WEEK;
            time += Date.WEEK; // add one week
        }
        var date2 = new Date(time);
        field.value = date2.print("%d.%m.%Y");
    }
    Calendar.setup({
        inputField     :    "f_date_a",
        ifFormat       : "%d.%m.%Y",
        button         : "cal-button-1",
        align          : "Tr",
        onUpdate       :    catcalc
    });
    Calendar.setup({
        inputField     :    "f_calcdate",
        ifFormat       :    "%d.%m.%Y",
        button         : "cal-button-2",
        align          : "Tr"    
    });