WhereTZ: fast and precise timezone lookup

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


  • quite precise: uses timezone-boundary-builder, which takes source data from OpenStreetMap;
  • (relatively) 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 timezone-boundary-builder dataset 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.

Known problems

  • 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 ODbL.

Code license is usual MIT.