I was thinking that hCalendar might be helpful for helping with timezones. The basic idea, just like, ecmanaut says, is to send the zone information in GMT and let the browser do the conversion. So I’m thinking if we use the microformat for dates, hcalendar, then the date gets formatted as,
<abbr class="dtstart" title="2006-05-01T12:15:03.0Z">5:15am</abbr>
where the “title” attribute is machine readable and in GMT, and the body is human readable and in, presumably, the time zone of the page author. All that’s needed is a script (or greasemonkey plugin) like this one that walks the DOM, finds these hCalendar fragments, and replaces the time in the users timezone into the human-readable part of the date. So,
Ok… but there are a couple of problems. The first is the formatting has changed. The resolution to this would be to write a function that deduced the format from the example, and then fills in that format with the local timezone. Seems doable, at least in a way that works 80% of the time (and fails gracefully with a generic date format). The second is that the intent of the time has, in fact, been changed a little bit. The user needs to know that this has happened (the greenish background is a hint at that), and needs to be able to see the original string to compare. The user might also prefer to see the data formatted as the author intended, but to be able to hover over the date and see it transformed into his own timezone. This also seems doable.