var plugJs={
  stamp:0,
  tid:1,
  stampnow:'<?php echo NOW_TIME;?>',//统一开始时间戳
  intervalTime:function(){
    if(plugJs.stamp > 0){
      var day = Math.floor(plugJs.stamp / (60 * 60 * 24));
      var hour = Math.floor(plugJs.stamp / (60 * 60)) - (day * 24);
      var minute = Math.floor(plugJs.stamp / 60) - (day * 24 * 60) - (hour * 60);
      var second = Math.floor(plugJs.stamp) - (day * 24 * 60 * 60) - (hour * 60 * 60) - (minute * 60);
      if (day <= 9) day = '0' + day;
      if (hour <= 9) hour = '0' + hour;
      if (minute <= 9) minute = '0' + minute;
      if (second <= 9) second = '0' + second;
      jQuery('.t_h_'+plugJs.tid).html(hour);
      jQuery('.t_m_'+plugJs.tid).html(minute);
      jQuery('.t_s_'+plugJs.tid).html(second);
      plugJs.stamp--;
      setTimeout('if(typeof(plugJs.intervalTime) == "function"){plugJs.intervalTime();}',1000);
    }
  },
  timer:function (stampend,tid){
    plugJs.stamp = parseInt(stampend)-parseInt(plugJs.stampnow);//剩余时间戳
    setTimeout('if(typeof(plugJs.intervalTime) == "function"){plugJs.intervalTime();}',1000);
  }
};
jQuery(document).ready(function(){
  var stampend = parseInt(jQuery('.countdown_1').attr('data-time'));//灵活读取表里的结束时间戳
  plugJs.timer(stampend,'1');
});

<div class="time countdown_1" data-time="1449429731">
  <span class="t_h_1">00</span>
  <i class="lay_line">:</i>
  <span class="t_m_1">00</span>
  <i class="lay_line">:</i>
  <span class="t_s_1">00</span>
</div>
<div class="time countdown_2" data-time="1449456731">
  <span class="t_h_2">00</span>
  <i class="lay_line">:</i>
  <span class="t_m_2">00</span>
  <i class="lay_line">:</i>
  <span class="t_s_2">00</span>
</div>