请参见以下示例→ ;

这应该可以解决问题:

$('#mem_dur').change(function(e) {
    var tVal = $(this).find('option:selected').val()[0],
        tDate = new Date(),
        mDate, mDateMS, mDateStr
        oneMonth = 31 * 86400000;

    mDateMS = tDate.getTime() + tVal * oneMonth;
    mDate = new Date(mDateMS);
    mDateStr = String(mDate.getFullYear()) + 
               ((mDate.getMonth() < 9) ? "0" + (mDate.getMonth() + 1) : mDate.getMonth() + 1) + 
               ((mDate.getDate() < 10) ? "0" + mDate.getDate() : mDate.getDate());

    mDateStr = (isNaN(parseInt(mDateStr,10))) ? "" : mDateStr;

    $('#sub_enddate').val(mDateStr);
});