FW: Fingerprinting timezones

Conrad Irwin conrad.irwin at gmail.com
Wed Aug 10 18:17:08 UTC 2011

Thank you all for your replies (including those received off-list).
I'm very encouraged, and will have a go at doing this when I next find
a spare weekend :).


On Tue, Aug 9, 2011 at 7:05 AM, Mark Davis ☕ <mark at macchiato.com> wrote:
> If someone is using a particular version of the TZ database, you can probe
> about 5-6 past dates to discover the zone id (of course, you can't
> distinguish the zones that have the same rules). We do that in browsers,
> where there isn't an API to get the zone.
> Where the browser isn't using the TZ database (eg windows), you'll just get
> an approximation.
> Mark
> — Il meglio è l’inimico del bene —
> On Mon, Aug 8, 2011 at 21:16, Olson, Arthur David (NIH/NCI) [E]
> <olsona at dc37a.nci.nih.gov> wrote:
>> 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?
>> Conrad

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