var tomorrow = document.createTextNode(moment().add(7,'days').format('YYYY MM DD')+' ');
document.getElementById('selected').appendChild(tomorrow);

var picker;
picker = new Pikaday(
        {
            field: document.getElementById('datepicker'),
            firstDay: 1,
            minDate: new Date('2000-01-01'),
            maxDate: new Date('2020-12-31'),
            yearRange: [2000, 2020],
            onSelect:
            i18n: {
                previousMonth: 'Previous Month',
                nextMonth: 'Next Month',
                months: ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'],
                weekdays: ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'],
                weekdaysShort: ['日', '一', '二', '三', '四', '五', '六']
            }
        });

如上代码，tomorrow获取的当前时间加七天后的日期，picher获取的是当前日期，在Pikaday内部调用onSelect回调函数 与当前日期作比较,就是tomorrow+7后与当前选定日期，如果当前选定的日期小于tomorrow+7后的日期则alert一条警告消息； onSelect:function(){....?}该怎么判断呢