I started to write a response, but it grew too long, so I wrote this blog post instead: http://tech.bluesmoon.info/2013/08/dont-guess-at-timezones-in-javascript

I started to write a response, but it grew too long, so I wrote this blog post instead: http://tech.bluesmoon.info/2013/08/dont-guess-at-timezones-in-javascript.html http://tech.bluesmoon.info/2013/08/dont-guess-at-timezones-i... tl;dr: the timezone you use depends not just on the user and their environment, but also the event and its duration.