function counter() {
    with (document.reservation) {

      // tarkistetaan onko kentät valittu
      var sd = document.reservation.saapuminen_d.value;
      var sm = document.reservation.saapuminen_m.value;
      var sy = document.reservation.saapuminen_y.value;
      var ld = document.reservation.lahto_d.value;
      var lm = document.reservation.lahto_m.value;
      var ly = document.reservation.lahto_y.value;

      if (sd != "" && sm != "" && sy != "" && ld != "" && lm != "" && ly != "") {
	var date1 = new Date(sy + "/" + sm + "/" + sd);
	var date2 = new Date(ly + "/" + lm + "/" + ld);
        var days = days_between(date1, date2);
	if (days > 7) {
		alert("You may reserve max 7 nights. Voit varata enintään 7 yötä.");
		document.reservation.yolkm1.value = days;
		document.reservation.yolkm.value = days;
		return false;
	}else{
		document.reservation.yolkm1.value = days;
		document.reservation.yolkm.value = days;
        	return true;
	}
      }
    }
  }

function days_between(date1, date2) {

    // The number of milliseconds in one day
    var ONE_DAY = 1000 * 60 * 60 * 24;

    // Convert both dates to milliseconds
    var date1_ms = date1.getTime();
    var date2_ms = date2.getTime();

    // Calculate the difference in milliseconds
    var difference_ms = Math.abs(date1_ms - date2_ms);
    
    // Convert back to days and return
    return Math.round(difference_ms/ONE_DAY);

}