js格式化日期、計算時間差

var startTime ="20170501"

var endTime = "20170430"

var pattern = /(\d{4})(\d{2})(\d{2})/;

var startTime_f = startTime.replace(pattern, '$1-$2-$3')

var endTime_f = endTime.replace(pattern, '$1-$2-$3')

//轉化後的時間格式 startTime_f=“2017-05-01”  endTime_f="2017-04-30"

var time1 = Date.parse(new Date(startTime_f));

var time2 = Date.parse(new Date(endTime_f));

var nDays = Math.abs(parseInt((time2 - time1)/1000/3600/24));