倒计时中的天数

// 详细日期可忽略为 new Date("2021")
let endTime =  new Date("2021/1/1 00:00:00").getTime() 

let restTime = endTime   -  new Date().getTime()

day = Math.floor(restTime/1000/60/60/24);  
hour = Math.floor(restTime/1000/60/60);  
min = Math.floor(restTime/1000/60);  
sec = Math.floor(restTime/1000);