Fast and precise time zone by geo coordinates lookup
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WhereTZ: fast and precise timezone lookup

WhereTZ is a small gem for lookup of timezone by georgraphic coordinates.


  • quite precise: uses prominent tz_world_map timezones database;
  • quite fast: 0.1-0.2 sec for lookup in worst cases and almost immediate lookup for best cases;
  • no calls to external services, works without Internet connection;
  • no keeping some 50 Mb datafiles in memory or reading them from disk for each call;
  • can return timezone name string or TZInfo::Timezone.


Install it with your usual routines (Gemfile or gem install) as wheretz gem. Then:

require 'wheretz'

WhereTZ.lookup(50.004444, 36.231389) # (lat, lng) order
# => 'Europe/Kiev'

WhereTZ.get(50.004444, 36.231389)
# => #<TZInfo::DataTimezone: Europe/Kiev>

# you should have tzinfo gem installed, wheretz doesn't list it as dependency

From command-line, after gem installed:

wheretz 50.004444,36.231389
# => Europe/Kiev

How it works

  1. Latest version of tz_world_map is converted into 400 data/*.geojson files;
  2. Each of those files corresponds to one timezone; filename contains timezone name and bounding box (min and max latitude and longitude);
  3. On each lookup WhereTZ first checks provided coordinates by bounding boxes, and if only one bbox (extracted from filename) corresponds to them, returns timezone name immediately;
  4. If there's several intersecting bounding boxes, WhereTZ reads only relevant timezone files (which are not very large) and checks which polygon actually contains the point.
  5. Added in 0.0.2 If point occures outside any of polygons, find the closest one (it's last resort for really rare cases and slower than other approaches).

Known problems

  • On "bounding box only" check, some points deeply in sea (and actually belonging to no timezone polygon) can be wrongly guessed as belonging to some timezone;
  • Loading/unloading .geojson files can be uneffective when called multiple times; future releases will provide option for preserve data in memory, or for mass lookup of points;
  • You should note that gem has ≈50 MiB of datafiles inside;
  • Data conversion performed by pretty ugly script (instead of Rake task as it should be).


Victor Shepelev


Data license is described at tz_world_map and reads like:

To the extent possible under law, Eric Muller has waived all copyright and related or neighboring rights to the maps (comprising the shapefiles, the web pages describing them and the scripts and data used to build them). This work is published from the United States of America.

Note that this does not affect the rights others may have. I am not qualified to determine whether such rights exist.

Code license is usual MIT.