setTimeout延时多久，不一定到时间后就会立即执行，所以需要来修正 setTimeout的时间，使其相对时间是准确的。

    1. let period = 60 * 1000 * 60 * 2
    2. let startTime = new Date().getTime()
    3. let count = 0
    4. let end = new Date().getTime() + period
    5. let interval = 1000
    6. let currentInterval = interval
    7. function loop() {
    8.   count++
    9.   // 代码执行所消耗的时间
    10.   let offset = new Date().getTime() - (startTime + count * interval);
    11.   let diff = end - new Date().getTime()
    12.   let h = Math.floor(diff / (60 * 1000 * 60))
    13.   let hdiff = diff % (60 * 1000 * 60)
    14.   let m = Math.floor(hdiff / (60 * 1000))
    15.   let mdiff = hdiff % (60 * 1000)
    16.   let s = mdiff / (1000)
    17.   let sCeil = Math.ceil(s)
    18.   let sFloor = Math.floor(s)
    19.   // 得到下一次循环所消耗的时间
    20.   currentInterval = interval - offset 
    21.   console.log('时：'+h, '分：'+m, '毫秒：'+s, '秒向上取整：'+sCeil, '代码执行时间：'+offset, '下次循环间隔'+currentInterval) 
    22.   setTimeout(loop, currentInterval)
    23. }
    24. setTimeout(loop, currentInterval)