Fake time zone for web app testing [duplicate]

I have a web app and some people in Asia are seeing some weird time zone issues with the dates in the program. Is there a way to fake chrome or firefox or IE to pretend like my computer's timezone is like theirs so I can debug the issues easily?

Sure: launch the browser from the command line with a customized TZ environment variable instead of the way you normally run it:

TZ=Japan firefox &

Make sure firefox isn't already running when you launch it like this because if it is already running then it will probably just open a new window under the existing instance of the browser instead of starting a new instance.