JSFiddle - React, Tailwind, and code Playground

HTML

<form name="disponibilidad">	
					<div class="arrival">
							<span>Start</span>
							<label>
								<select name="llegada-dia">
                  <option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option><option value="5">5</option><option value="6">6</option><option value="7">7</option><option value="8">8</option><option value="9">9</option><option value="10">10</option><option value="11">11</option><option value="12">12</option><option value="13">13</option><option value="14">14</option><option value="15">15</option><option value="16">16</option><option value="17">17</option><option value="18">18</option><option value="19">19</option><option value="20">20</option><option value="21">21</option><option value="22">22</option><option value="23">23</option><option value="24">24</option><option value="25">25</option><option value="26">26</option><option value="27">27</option><option value="28" selected="selected">28</option><option value="29">29</option></select>
							</label>
							<label>
								<select name="llegada-mes">
									<option value="" selected=""></option>
									<option value="0">jan</option>
                  <option value="1">feb</option>
                  <option value="2">mar</option>
                  <option value="3">apr</option>
                  <option value="4">may</option>
                  <option value="5">jun</option>
                  <option value="6">jul</option>
                  <option value="7">ago</option>
                  <option value="8">sep</option>
                  <option value="9">oct</option>
                  <option value="10">nov</option>
                  <option value="11">dec</option>		
								</select>
							</label>
							<label>
								<select name="llegada-ano">
									<option value="" selected=""></option>
									<option value="2013">2013</option>
                  <option value="2014">2014</option>
                  <option...

CSS

*{ margin: 5px;}
div span{
    
    display:block;
}

JavaScript

$(function(){
    months = ['jan','feb','mar','apr','may','jun','jul','ago','sep','oct','nov','dec'];
		
    /* Cachear selects */
		var $ld = $('select[name=llegada-dia]');
		var $lm = $('select[name=llegada-mes]');
		var $ly = $('select[name=llegada-ano]');
		var $sd = $('select[name=salida-dia]');
		var $sm = $('select[name=salida-mes]');
		var $sy = $('select[name=salida-ano]');
		
    var manyDays = function( month, year ){
			//console.log('M/Y:'+month,year);
      // +1 Otherwise you get last month
      var days_in_month = new Date(year, month+1, 0);
      //console.log("Date:"+days_in_month);
      return days_in_month.getDate();
    }
    
    var paintCals = function( day, month , year ){
			
      if(day == '') day = 1;
			if(month == '') month = 0;
			if(year == '' ) year = 2013;
			
      // needed to avoid Feb 31
      var check_fecha = new Date( year, month );			
			check_mes = check_fecha.getMonth();
      check_anyo = check_fecha.getFullYear();
      var check_dias_mes = manyDays( check_mes, check_anyo );
      
      if(day > check_dias_mes)
        day = check_dias_mes;
       
      var fecha = new Date( year, month , day );			
			var dia = fecha.getDate();
			var mes = fecha.getMonth();
			var anyo = fecha.getFullYear();
			//console.log('D/M/Y:'+dia,mes,anyo);
      //console.log('Day:'+day,"Dia:"+dia);
      
      var dias_mes = manyDays( mes,anyo );
      
      var next_fecha = fecha;
      next_fecha.setDate(next_fecha.getDate()+1);
      
      next_fecha.setFullYear(fecha.getFullYear() + (mes == 12 ? 1 : 0));
      
			var next_dia = next_fecha.getDate();
			var next_mes = next_fecha.getMonth();
			var next_anyo = next_fecha.getFullYear();
			//console.log('nD/nM/nY:'+next_dia,next_mes,next_anyo);
      
      
      // Otherwise you get last month
      var next_dias_mes = manyDays( next_mes, next_anyo ) ;
      
      /* LOG*/
      $('#log').empty().append('Mes: ' + mes).append('<br>'); 
      $('#log').append('Next mes: '+...