设置一个定时器，每秒计算一次。（在这之前清除一次防止重复调用setInterval()函数）

引入的时候需要全局引入js文件，并添加data-pjax，否则切换到别的页面就失效了

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<span class="item-count" id="runtimeshow" data-publishdate="2020-09-30T16:00:00.000Z"></span>
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/*
文件     :myUtils.js
时间     :2021-02-10 07:00:36
作者     :Karcyril
*/


/**添加网站运行时间 */

function myGetRuntime(d) {
  const dateNow = new Date()
  const datePost = new Date(d)
  const dateDiff = dateNow.getTime() - datePost.getTime()
  const minute = 1000 * 60
  const hour = minute * 60
  const day = hour * 24

  let result = {}

  const dayCount = Math.floor(dateDiff / day)
  const hourCount = Math.floor(dateDiff % day / hour)
  const minuteCount = Math.floor(dateDiff % day % hour / minute)
  /**最后余下的为毫秒数 除以1000得到秒数 */
  const secondCount = Math.floor(dateDiff % day % hour % minute / 1000)

  result.day = dayCount
  result.hour = hourCount
  result.minute = minuteCount
  result.second = secondCount

  return result
}

var mTimer

(function () {
  const $runtimeCount = document.getElementById('runtimeshow')
  if ($runtimeCount) {
    const publishDate = $runtimeCount.getAttribute('data-publishDate')
    clearInterval(mTimer)
    mTimer = setInterval(function () {
      const runtimeDate = myGetRuntime(publishDate)
      $runtimeCount.innerText = '本站已运行 ' + runtimeDate.day + '天' +
 runtimeDate.hour + '小时' + runtimeDate.minute + '分钟' +
 (runtimeDate.second < 10 ? ('0' + runtimeDate.second) : runtimeDate.second) +
 '秒'
    }, 1000)

  }
})()