Fingerprinting timezones

[Conrad Irwin]

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