I have read loads of posts and howto's and virtually every one I have found says "can't be done" or "can't be done reliably" or "not cross browser compatible". All the articles I have read basically fall back to the suggestion of "make a user setting".
However when I login to cpanel, it automatically detects my timezone. When I visit most calendar software apps they automatically convert to my timezone. When I use my webinar software, the webinar times are automatically converted for the user when they visit the page - no login required - seems to work in IE, FF, Chrome, Opera, Edge, and a few offshoot browsers.
So without suggesting it can't be done, its not reliable, I have to use offsets (my cookies show they are actually setting America/Denver NOT an offset), that its not cross browser compatible, etc.... can someone tell me HOW they are doing this?