JSFiddle - React, Tailwind, and code Playground

HTML

<select class='select1'>
    <option selected disabled>Select One</option>
    <option>Select this</option>
    <option>or this</option>
</select>
<select class='select2'>
    <option selected disabled>Select One</option>
</select>

JavaScript

Months = {
    january:31,
    february:28,
    March:31,
    april:30,
    may:31,
    june:30,
    july:31,
    august:31,
    september:30,
    october:31,
    november:30,
    december:31
}
$('.select1').html('');
$.each(Months,function(month,days){
    $('.select1').append('<option>'+month+'</option>');
});
$('.select1').change(function() {
    selected = $('.select1').val();
    $('.select2').html('');
    $.each(Months,function(month,days) {
        if (month == selected) {
            for (i=1;i<=days;i++) {
                $('.select2').append('<option>'+i+'</option>');
            }
        }
    });
});