How does Gmail know my local time zone (therefore ignoring the time zone of the Tor exit node) and what else can it see?

[Matthew]

On 05/09/10 21:11, Geoff Down wrote:
> On Sun, 05 Sep 2010 19:55 +0100, "Matthew"<pumpkin at cotse.net>  wrote:
>>    Hello,
>>
>> I have yet another question that relates to the effectiveness of Tor.
>>
>> Gmail (and therefore presumably other webmail operators) knows my
>> computer's time zone.  It does not know the time per se but the "time
>> zone"
>> as set (in Ubuntu) through clicking on the clock, selecting preferences,
>> then choosing location.
>>
>> Obviously this ignores the time (based on the location) of the Tor exit
>> node.
>>
>> I do not know how Gmail knows my computer's time zone, and, in which
>> case,
>> what other "local" information it can know.
>>
>> Does anyone know how Gmail can do this and what other information from
>> the
>> client computer can be viewed.  In other words, why can Gmail not, in
>> theory, also view the real local IP?
>>
>> Thanks.
> Did you select a time zone when you set up the account?
> I assume you are using Torbutton, which blocks Javascript being used to
> read your local clock.
> GD
>
AIUI, Gmail uses JavaScript to detect the time zone (but not the time) on 
the client machine.  When I use NoScript with Gmail as untrusted, Gmail 
cannot use JavaScript.  Changing the time zone settings (for example to 
something five hours behind my real time zone) does not then change the 
time at which e-mail appears to arrive in the Gmail inbox since this 
requires JavaScript which is not used since Gmail is considered untrusted.

However, since many websites do require JavaScript, whether or not one is 
using NoScript and / or TorButton, my question was:

If Gmail can get the time zone via JavaScript (when the client is using 
Tor) then why can it not get the "real" IP also via JavaScript (when the 
client is using Tor)?  I don't think it can get the real IP since I have 
used various tests including http://www.decloak.net/ and Tor with 
JavaScript does not reveal the real IP.  But why not?

Thanks.
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