// Calcula la fecha

	function calcNewDate(month,day,year,ciclo) {
        newday = eval(day) + (ciclo - 14)		
        newmonth = month + 1
        newyear = eval(year)
        var max

        for (var i = 0; i < 12; i++) {
          if (newmonth == 2 && (newyear % 4) == 0) {
            max = 29
          } else
            max = maxday[newmonth]
          	if (newday > max) {
            	newday = newday - max
            	newmonth = newmonth + 1

	            if (newmonth > 12) {
              		newyear = newyear + 1
	              	newmonth = 1
            	}
          	}
          else
            break
        }
        var datestring = newday + " de " + monthname[newmonth] 
        return datestring
	}



	// calcula fechas
	function calc(form) {
        day = form.day.value

        year = form.year.value
		desde = (eval(form.ciclo.value) - 3)
		hasta = (eval(form.ciclo.value) + 1)
		
        monthnum = form.month.selectedIndex
        if (diavalido(day)) {
          if (aniovalido(year)){
            form.fertildesde.value = calcNewDate(monthnum,day,year,desde)
			form.fertilhasta.value = calcNewDate(monthnum,day,year,hasta)
          }
        } 
	}