function getTime(){ 
    str = "当前系统时间：" 
    var p = document.getElementById("sy_time"); 
    time = new Date(); 
    year = time.getFullYear(); 
    month = time.getMonth() + 1; 
    day = time.getDate(); 
    hour = time.getHours(); 
    minutes = time.getMinutes(); 
    seconds = time.getSeconds(); 
    str = str + year +"-"+ month +"-"+ day + " " +hour+":"+minutes+":"+seconds; 
    p.innerText = str; 
    setTimeout(getTime,1000); 
  } 
  window.onload = function(){ 
    getTime(); 
  } 

<!-- 页面调用代码 -->
<p id = "sy_time"></p>