Make ICal an Application and check the timezone on start

lib/ical/application.ex

@@ -0,0 +1,42 @@
+defmodule ICal.Application do
+  @moduledoc false
+
+  use Application
+  require Logger
+
+  @impl Application
+  def start(_type, _args) do
+    Calendar.get_time_zone_database()
+    |> check_timezone_availability()
+
+    Supervisor.start_link([], strategy: :one_for_one, name: ICal.Application)
+  end
+
+  defp check_timezone_availability(Calendar.UTCOnlyTimeZoneDatabase) do
+    cond do
+      Code.ensure_loaded?(Tz) ->
+        Logger.info(
+          "ICal has registered the Tz time zone database to ensure full function. Consider adding this to your app config: config :elixir, :time_zone_database, Tz.TimeZoneDatabase"
+        )
+
+        Calendar.put_time_zone_database(Tz.TimeZoneDatabase)
+        :ok
+
+      Code.ensure_loaded?(TimeZoneInfo) ->
+        Logger.info(
+          "ICal has registered the TimeZoneInfo time zone database to ensure full function. Consider adding this to your app config: config :elixir, :time_zone_database, TimeZoneInfo.TimeZoneDatabase"
+        )
+
+        Calendar.put_time_zone_database(TimeZoneInfo.TimeZoneDatabase)
+        :ok
+
+      true ->
+        Logger.error(
+          "No timezone database is instealled, and neither the Tz nor TimeZoneInfo libraries wer found. Please install one of those as per the ICal documentation to ensure that ICal works correctly. Without a timezone database, no calendaring data with timezones will parse correctly."
+        )
+    end
+  end
+
+  # there is a non-default timezone database installed, so ICal is good to go.
+  defp check_timezone_availability(_), do: :ok
+end

mix.exs

@@ -19,6 +19,14 @@ defmodule ICal.Mixfile do
     ]
   end
 
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {ICal.Application, []}
+    ]
+  end
+
   defp elixirc_paths(:test), do: ["lib", "test/support", "test/data"]
   defp elixirc_paths(_), do: ["lib"]