Lat/long to timezone mapper in Dart. Does not require web services or data files
The "lat/long to timezone polygon mapping" is hardcoded, and we hope this rarely changes, but the changes to offsets and daylight savings changeover dates etc. (which are more frequent) are taken care of by your system libraries and so these are automatically kept up-to-date. From time to time, someone updates the files with the latest timezone polygons, but these rarely change...I think the most recent change is the Crimean peninsular.
This Dart library is port of the original project
available at https://github.com/drtimcooper/LatLongToTimezone. Ported it to Dart to make it easily available on pub.dev. In order to support updates from the upstream version, the formatting was changed as little as possible (specifically not adding control flow braces or truncating lines at 80 characters).
See the example at pub.dev.