js time calculation

<script>


function DateAdd(interval, number, date) {
    switch (interval) {
    case "y": {
        date.setFullYear(date.getFullYear() + number);
        return date;
        break;
    }
    case "q": {
        date.setMonth(date.getMonth() + number * 3);
        return date;
        break;
    }
    case "m": {
        date.setMonth(date.getMonth() + number);
        return date;
        break;
    }
    case "w": {
        date.setDate(date.getDate() + number * 7);
        return date;
        break;
    }
    case "d": {
        date.setDate(date.getDate() + number);
        return date;
        break;
    }
    case "h": {
        date.setHours(date.getHours() + number);
        return date;
        break;
    }
    case "m": {
        date.setMinutes(date.getMinutes() + number);
        return date;
        break;
    }
    case "s": {
        date.setSeconds(date.getSeconds() + number);
        return date;
        break;
    }
    default: {
        date.setDate(date.getDate() + number);
        return date;
        break;
    }
    }
}

var now = new Date('2018-02-27 00:00:00');
// 加五天.
var newDate = DateAdd("d", 5, now);
alert(newDate.toLocaleDateString())
// 加两个月.
newDate = DateAdd("m", 2, now);
alert(newDate.toLocaleDateString())
// 加一年
newDate = DateAdd("y", 1, now);
alert(dateFtt('yyyy-MM-dd hh:mm:ss',newDate))
// 减1秒
newDate = DateAdd("s", -1, now);
alert(dateFtt('yyyy-MM-dd hh:mm:ss',newDate))

function dateFtt(fmt,date)   
{ //author: meizz   
  var o = {   
    "M+" : date.getMonth()+1,                 //月份   
    "d+" : date.getDate(),                    //日   
    "h+" : date.getHours(),                   //小时   
    "m+" : date.getMinutes(),                 //分   
    "s+" : date.getSeconds(),                 //秒   
    "q+" : Math.floor((date.getMonth()+3)/3), //季度   
    "S"  : date.getMilliseconds()             //毫秒   
  };   
  if(/(y+)/.test(fmt))   
    fmt=fmt.replace(RegExp.$1, (date.getFullYear()+"").substr(4 - RegExp.$1.length));   
  for(var k in o)   
    if(new RegExp("("+ k +")").test(fmt))   
  fmt = fmt.replace(RegExp.$1, (RegExp.$1.length==1) ? (o[k]) : (("00"+ o[k]).substr((""+ o[k]).length)));   
  return fmt;   
}
</script>