I don't think you will be successful at changing the user regional settings from the browser since that would require you to write "real code" as a active-x control.
But here is an idea to research: use IP geolocation.
Find out if there is a jsonp based API on the web that gives you details about the IP address the user is coming from. The goal would be for you to include JavaScript on the user's page that calls the API (which hopefully returns the IP's location and
time zone) and then match that time-zone with the user's time zone. If different, then do the appropriate thing. Which could be: prompt the user to check and set the time zone. Or maybe use the timezone returned by the webservice.
I did a quick google search and there are services out there.
Good luck.
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Paul
Sent from mobile device.