{"id":36,"date":"2008-06-21T16:12:20","date_gmt":"2008-06-21T08:12:20","guid":{"rendered":"http:\/\/blog.lolyco.com\/sean\/?p=36"},"modified":"2008-06-21T23:14:16","modified_gmt":"2008-06-21T15:14:16","slug":"if-modified-since-date-formats-in-firefox-and-ie7","status":"publish","type":"post","link":"https:\/\/blog.lolyco.com\/sean\/2008\/06\/21\/if-modified-since-date-formats-in-firefox-and-ie7\/","title":{"rendered":"If-Modified-Since date formats in Firefox and IE7"},"content":{"rendered":"

Everybody<\/strong> who has a webserver wants to use less bandwidth<\/strong>. A webserver<\/strong> sends out a lot of copies of the same information<\/strong>. Stylesheets, images, static HTML pages, javascript files. Many of these rarely<\/strong>, if ever,\u00a0 change<\/strong> from one visit by a browser to the next. Fortunately<\/strong>, there are a few schemes implemented in HTTP<\/strong> that help save bandwidth<\/strong>.<\/p>\n

I’ve been writing<\/strong> a webserver recently, so I’ve become more acquainted with HTTP features<\/strong> than I might ever have wanted to. Don’t ask me why<\/strong> I’m writing a webserver, I’m beginning to wonder<\/strong> myself. It seemed like a good idea<\/strong> at the time I started it.<\/p>\n

\"Apache<\/a>When I first started watching Apache<\/strong><\/a> access logs<\/strong> (this is a kind of mental illness<\/strong>, I can’t recommend it), I noticed 304<\/strong> responses for some popular files. HTTP 304 is ‘Not Modified<\/strong><\/a>‘. Most browsers cache<\/strong> files they download from webservers, and don’t download<\/strong> them again if they haven’t changed. The way they do this is by conditionally<\/strong> fetching content from the webserver.<\/p>\n

\"Firefox<\/a>Firefox<\/strong><\/a> had a super handy extension called ‘LiveHTTPHeaders<\/strong>‘ that allows you to see exactly what messages<\/strong> your browser exchanges with the web server. Even if you don’t have a direct use for it, the conversations<\/strong> between your browser and web servers can tell you a lot<\/strong> about how \"Gnome's<\/a>the Web works. Last time I looked, this extension wasn’t available<\/strong> in Firefox 3. Epiphany<\/strong><\/a> has something very similar, so I’ve been using that browser while I’m developing<\/strong> my webserver.\"Wireshark\"<\/a> You can read the HTTP conversations by using a tool like WireShark<\/a><\/strong> too, a very powerful<\/strong> and complex network analysis tool<\/strong>.<\/p>\n

A browser makes a conditional request from a webserver when it has an earlier copy<\/strong> of the object in its cache. To avoid<\/strong> the possibility of presenting out-of-date content<\/strong> to the user, the browser sends an ‘If-Modified-Since<\/a><\/strong>‘ header field, with the date the browser last downloaded the content. The Webserver checks this date against the date the content was last changed<\/strong> on the server. If the server content has changed, the content is sent as normal<\/strong>. If the content has not changed since the browser last downloaded it, the server makes a ‘304 Not Modified’ response<\/strong>, and the browser uses the cached content<\/strong>.<\/p>\n


\n\"GetJava<\/a>I noticed in the log file of my webserver a number of<\/p>\n

java.text.ParseException: Unparseable date<\/p><\/blockquote>\n

messages.\u00a0 I use Java<\/strong><\/a>‘s SimpleDateFormat class on the server for all sorts of different date parsing<\/strong>, but ran into some extra difficulties<\/strong> on this occasion. The difficulties arose because just one of the machines here uses IE7<\/strong> on Windows XP. This browser was sending a different format<\/strong> from all the other browsers:<\/p>\n