cquiroz / sbt-tzdb

sbt plugin to create custom timezone db files

Version Matrix


Build a custom time zone database for Scala.js applications.

sbt-tzdb is a code generation tool used to build a time zone db compatible with the Scala.js side of scala-java-time.

Its main purpose is to let users build a custom version of tzdb, which has the minimal data your application needs reducing the size of the application. This is fairly important for Scala.js applications.

What does it do?

The plugin will download a time zone data package from IANA TimeZone Database, then it will parse the database and build suitable code that scala-java-time can use to support the timezone base operations.

This should only happen once on your build. If you need to regenerate the db, e.g. if the version has changed or your filter is different, just clean your project


The full timezone databes is fairly large containing historical records for all time zones. A typical application doesn't need so many timezones, however the full database must be carried due to the properties of the java.time API, which search timezones with strings. This doesn't let the Scala.js optimizer remove unnecessary zones.

However, it is legal to have a smaller database with only the time zones of your interest. As it is impossible to publish all possible combinations, this plugin lets you generate dynamically the database for your application.

By restricting the avaliable timezones you can reduce your js size up to several megabytes on fastOptJS.

Usage Instructions

add the plugin dependency to project/plugins.sbt for sbt 1.1.x

addSbtPlugin("io.github.cquiroz" % "sbt-tzdb" % "0.3.1")

The plugin currently supports sbt 1.1.x.

The you need to enable the plugin in your project e.g.:


Note that the plugin should be only enabled for js projects but it is not enforced. For cross projects you should do:

lazy val lib = crossProject(JVMPlatform, JSPlatform)

lazy val libJVM = lib.jvm
lazy val libJS  = lib.js.enablePlugins(TzdbPlugin)

This only makes sense if you add scala-java-time as a dependency

libraryDependencies ++= Seq(
  "io.github.cquiroz" %%% "scala-java-time" % "2.0.0-RC1"

Main Tasks

The plugin attaches to the build and adds a custom code generation task which will build a compatible timezone

Configuration settings

  • dbVersion: Lets you specify what tzdb version you want to use. It defaults to latest
  • zonesFilter: A function to filter what timezones are included. By default all zones are included but to reduce the size you should specify only the ones you need, e.g.:
zonesFilter := {(z: String) => z == "America/Santiago" || z == "Pacific/Honolulu"},


  • This is still an experimental plugin, use with care