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 value="2015">2015</option><option value="2016">2016</option><option value="2017">2017</option><option value="2018">2018</option><option value="2019">2019</option><option value="2020">2020</option>		
								</select>
							</label>
							<input name="llegada"...

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 ){
			var fecha = new Date(year, (month+1) , 0);
			return fecha.getDate();
		}
		var paintCals = function( day, month , year ){
			if(day == '') day = 1;
			if(month == '') month = 0;
			if(year == '' ) year = 2013;
			//month = month -1;
			var fecha = new Date( year, month , day );			
			var dia = fecha.getDate();
			var mes = fecha.getMonth();
			var anyo = fecha.getFullYear();
			var dias_mes = manyDays( mes,anyo );
			
			var next_fecha = fecha;
            
			next_fecha.setDate(next_fecha.getDate() + 1); 

            
			var next_dia = next_fecha.getDate();
			var next_mes = next_fecha.getMonth();
			var next_anyo = next_fecha.getFullYear();
            console.log(next_dia, next_mes, next_anyo);
			var next_dias_mes = manyDays( next_mes, next_anyo ) ;
            
            /* LOG*/
            $('#log').empty().append('Mes: ' + mes).append('<br>'); 
            $('#log').append('Next mes: '+ next_mes).append('<br>');
            
            $('#log').append('Max dias mes:' + (dia == dias_mes ? 1 : 0)).append('<br>');
            $('#log').append('Fecha: ' + fecha).append('<br>');
            $('#log').append('Siguiente: ' + next_fecha);
			/* Actualizar dias entrada */
			$ld.empty();
			for(var tmpdia = 1; tmpdia <= dias_mes; tmpdia++){
				var doption = $('<option>').attr( 'value',tmpdia )
										   .text( tmpdia );
				if(dia == tmpdia && dia != ''){
					doption.attr('selected', 'selected');
				}
				$ld.append(doption);
			}
			/* Actualizar dias salida */
			$sd.empty();
			for(var tmpdia = next_dia; tmpdia <= next_dias_mes;...