function setMonthSelect(select){
    const options = select.options,
          selectedOption = options[options.selectedIndex],
          year = selectedOption.value,
          months = selectedOption.dataset.months,
          monthStart = months == 9 ? 4 : 1;

    document.querySelectorAll('[data-control]').forEach(sel =>{
        removeOptions(sel);
        sel.disabled = year ? false : true;
    });

    const monthSelect = document.getElementById('month');
    for(let i = 0; i < Number(months); i++) {
        const month = new Date(year, monthStart+i, 0).getMonth()+1;
        monthSelect.append(createOption(month,'月'));
    }

}


function setDaySelect(month){
    const yearSelect = document.getElementById('year'),
          year = yearSelect.value,
          countMonths = yearSelect.options[yearSelect.options.selectedIndex].dataset.months;

          dayStart = (countMonths == 9 && month == 4) ? 2 : 1;

    if(!year|| !month) return ;

    const daySelect = document.getElementById('day'),
          lastDay  = (countMonths == 4 && month == 4) ? 1 : new Date(year, month, 0).getDate();
    removeOptions(daySelect);

    for(let i = dayStart; i <= lastDay; i++) daySelect.append(createOption(i,'日'));

}

function createOption(val,text){
    const option = document.createElement('option');
    option.textContent = val + text;
    option.value = val;

    return option
}

function removeOptions(parent){
    parent.querySelectorAll('option:nth-child(n+2)').forEach(option => option.remove())
}