Comments on: If-Modified-Since date formats in Firefox and IE7

By: Sean

Sean — Tue, 23 Jun 2009 04:37:14 +0000

Thanks, Spenser. In retrospect, it’s not surprising at all. The If-Modified-Since technique (for example) is obviously more robust and more efficient if all servers use the same time standard. I think my surprise had more to do with my own ignorance of time standards and the idea that such an important standard should be based on a location ‘just up the road’ from where I was born!

By: spenser

spenser — Tue, 23 Jun 2009 04:15:34 +0000

There is a very good reason for the use of GMT. It is unambiguous, even when countries or regions decide to unadvisedly and unilaterally change the rules for calculating daylight savings time. For those who look at GMT regularly, they soon get the hang of making the required mental adjustment for timezone offsets as they browse logs.

By: Sean

Sean — Thu, 17 Jul 2008 07:41:10 +0000

Thanks very much for that pointer Gareth - that put me on the right path. My server was indeed sending out dates in a funky format - the Date.toString() format! It's close to the asctime() format mentioned in the IETF's draft HTTP/1.0 spec, but not quite the same. One last wrinkle that surprised me was the insistence on GMT:

All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception.

I've re-coded the If-modified-since behaviour of my webserver. Now it follows IETF's recommendation in RFC1123's Robustness Principle:

"Be liberal in what you accept, and conservative in what you send"

The non-IE browsers I was using must be designed along the lines of "When in Rome, do what the Romans do"!

By: Gareth Farrington

Gareth Farrington — Wed, 16 Jul 2008 21:40:10 +0000

I was writing some file serving code and I saw something like this.

The HTTP spec recommends that the client store the exact date/time sent from the server, in the format the server sent it. So Firefox does this and if your server sends timestamps in the Last-Modified header in some funky format the browser will spit it back out in that same format.

IE is using an RFC 1123 date format which is THE preferred format for all dates in HTTP headers. IE transformed the date sent from the server into the standard format. This can cause problems if the server and client clocks are not synchronized or if the server uses a non standard date format.

So the Firefox behaviors is safer but it returns illegal date strings. The IE behavior is less robust but its output is not illegal.

Check the server code to see if you are sending out last modified headers in the “Fri May 30 10:14:52 MYT 2008″ format (which is invalid)

By: Poon Poi Ming

Poon Poi Ming — Sat, 21 Jun 2008 14:59:02 +0000

Hi Seanie, I got utterly lost halfway. Too technical for me, but I guess the target audience of this article is more for the tech-savvy, LOL.