First to find out what is somebody's timezone I pick few recent trades and rying to fit them to my chart [so open and closing time fit to order level.]
Than in excell I just use another column, where I first format it to make sure it will display time, and than I write =X2+01:00, or X2+60 [where X2 is parallel cell with time value].
I think if you make a software, the easiest way is to create option to draw 10 recent trades just to see if they fit chart, and then make TZ adjustment, draw first 10 again, to see are they fit, and if all is fine, than one can go with full history.
Actually this may be possible that you keep all soft on your website, and what someone is downloading would be created by you .tpl file with all lines already marked, same as you generate different widgets, there may be an option to generate different tpl for different timezones.