index.js

document.writeln("<header class=\"header left\">\n");
document.writeln("  <div class=\"left nav\">\n");
document.writeln("    <ul>\n");
document.writeln("      <li class=\"nav_active\">\n");
document.writeln("        <i class=\"nav_1\"><\/i><a href=\"index.html\">首页<\/a>\n");
document.writeln("      <\/li>\n");
document.writeln("      <li><i class=\"nav_2\"><\/i><a href=\"carContrl.html\">车辆监控<\/a><\/li>\n");
document.writeln("      <li><i class=\"nav_3\"><\/i><a href=\"map.html\">地图界面<\/a><\/li>\n");
document.writeln("    <\/ul>\n");
document.writeln("  <\/div>\n");
document.writeln("  <div class=\"header_center left\">\n");
document.writeln("    <h2><strong>政务服务中心食品安全管理云平台<\/strong><\/h2>\n");
document.writeln("  <\/div>\n");
document.writeln("  <div class=\"right nav text_right\">\n");
document.writeln("    <ul>\n");
document.writeln("      <li><i class=\"nav_7\"><\/i><a href=\"static.html\">查询统计<\/a><\/li>\n");
document.writeln("      <li><i class=\"nav_8\"><\/i><a href=\"message.html\">信息录入<\/a><\/li>\n");
document.writeln("      <li><i class=\"nav_4\"><\/i><a href=\"table1.html\">表格界面<\/a><\/li>\n");
document.writeln("      <li ><span class=\"adminName\">周磊</span><span id=\"time\"></span><\/li>\n");
document.writeln("    <\/ul>\n");
document.writeln("  <\/div>\n");
document.writeln("<\/header>\n");
document.writeln("\n");

window.onload = function() {

	//定时器每秒调用一次fnDate()
	setInterval(function() {
		fnDate();
	}, 1000);
}

//js 获取当前时间

function fnDate() {
	var oDiv = document.getElementById("time");
	var date = new Date();
	var year = date.getFullYear(); //当前年份
	var month = date.getMonth(); //当前月份
	var data = date.getDate(); //天
	var hours = date.getHours(); //小时
	var minute = date.getMinutes(); //分
	var second = date.getSeconds(); //秒
	var time = year + "-" + fnW((month + 1)) + "-" + fnW(data) + " " + fnW(hours) + ":" + fnW(minute) + ":" + fnW(second);
	oDiv.innerHTML = time;
}
//补位 当某个字段不是两位数时补0

function fnW(str) {
	var num;
	str > 10 ? num = str : num = "" + str;
	return num;
}

index.html

<script src="js/header.js"></script>