js计算两个时间戳之间的天、时、分、秒间隔

 countdownTime为两个时间的差值

let runTime = parseInt(countdowmTime / 1000);
let day = Math.floor(runTime / 86400);
runTime = runTime % 86400;
let hour = Math.floor(runTime / 3600);
runTime = runTime % 3600;
let minute = Math.floor(runTime / 60);
runTime = runTime % 60;
let second = runTime;