$.fn.selectDate = function(){
    var minYear = 1900
    var maxYear = (new Date).getFullYear()
    var yearSel = document.getElementById('year')
    var monthSel = document.getElementById('month')
    var daySel = document.getElementById('days')

    for(var y = maxYear;y >= minYear;y--){
        var yearOpt = document.createElement('option')
        yearOpt.value = y
        yearOpt.innerHTML = y+'年'
        yearSel.appendChild(yearOpt)
    }

    $("#year").click(function(event){
        if(!$("#year option:selected").val()) return;
        removeOption(monthSel)
        addOption(12,'月',monthSel)
        removeOption(daySel)
    })

    $("#month").click(function(){
        removeOption(daySel)
        var year = $("#year option:selected").val()
        var month = $("#month option:selected").val()
        if(month==1 || month==3 || month==5 || month==7 || month==8 || month==10 || month==12){
            addOption(31,'日',daySel)
        }else if(month==4 || month==6 || month==9 || month==11){
            addOption(30,'日',daySel)
        }else if(month==2){
            if((year%4 == 0 && year%100 != 0 ) || (year%400 == 0)){
                addOption(29,'日',daySel)
            }else{	
                addOption(28,'日',daySel)
            }
        }
    })

    function addOption(num,unit,parent){
        //num:选项个数
        //unit:单位(年/月/日)
        //parent:父对象
        for(var index=1;index <= num;index++){
            var opt =document.createElement('option')
            $(opt).attr('value',index)
            if(index<10){index = '0'+index}
            $(opt).html(index+unit)
            $(parent).append(opt)
        }
    }
    
    function removeOption(parent){
        //parent:父对象
        var options = $(parent).find('option')
        for(var index = 1;index < options.length;index++){
            parent.removeChild(options[index])
        }
    }
}