FW: Fingerprinting timezones

Olson, Arthur David (NIH/NCI) [E] olsona at dc37a.nci.nih.gov
Tue Aug 9 04:16:02 UTC 2011

I'm forwarding this message from Conrad Irwin, who is not on the time zone mailing list. Those of you who are on the list, please direct replies appropriately.  --ado

-----Original Message-----
From: Conrad Irwin [mailto:conrad.irwin at gmail.com] 
Sent: Monday, August 08, 2011 7:59
To: tz at lecserver.nci.nih.gov
Subject: Fingerprinting timezones

Hello there,

Thank you for the timezone database - it is a phenomenal resource.

The problem I would like to solve is that of having to ask a user of a
web-app for their timezone. While there is no way to directly query
for the user's timezone, the javascript engine in their web-browser is
capable of telling me the offset from UTC at any date.

It seems like, by selecting some number of dates to query, I should be
able to gather enough information to uniquely identify any given
timezone (or at least make a very good guess for a large proportion of
the user-base).

Before I embarked on doing this, I wanted to ask if anyone has seen a
similar attempt in the past? Are there some "gotchas" that mean that
this isn't actually as feasible as I naïvely hope?


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