如题！
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看看HTML源代码就知道了，关键部分应该是：
- HTML code
<div id="ad_left" style="position:absolute;width:120px;top:184px;left:10px;display:none;"><a style="float:right;" href="#" onclick="javascript:this.parentNode.style.display='none';return false;">[关闭]</a><script type="text/javascript"> try { var pdate = "" + parent.eval("tinfo.pdate"); if (pdate) { pdate = new Date(pdate.replace(/-/g, "/")); var today = new Date(); showad = (today.getTime() - pdate.getTime()) / 1000 / 60 / 60 / 24 > 30; } } catch (ex) { };showad=true; try { var ad_left = ""; if (showad && screen.availWidth >= 1280) { ad_left += '<' + 'script type=\"text/javascript\">'; ad_left += 'google_ad_client = "pub-1076724771190722";'; ad_left += 'google_ad_slot =\"5633584852\";'; ad_left += 'google_ad_width = 120;'; ad_left += 'google_ad_height = 600;'; ad_left += '</' + 'script>'; ad_left += '<' + 'script type=\"text/javascript\"'; ad_left += 'src=\"http://pagead2.googlesyndication.com/pagead/show_ads.js\">'; ad_left += '</' + 'script>'; } document.write(ad_left); document.getElementById("ad_left").style.display = ""; } catch (e) { }</script></div>
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showad = (today.getTime() - pdate.getTime()) / 1000 / 60 / 60 / 24 > 30;
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同意一楼，通过算时间差来控制