Add advances_clock?/1

README.md

@@ -19,6 +19,7 @@
 * `TzExtra.new_resolved_datetime!/4`
 * `TzExtra.utc_datetime_range/3`
 * `TzExtra.round_datetime/3`
+* `TzExtra.advances_clock?/1`
 
 ### `TzExtra.countries_time_zones/0`
 

lib/compiler.ex

@@ -236,6 +236,22 @@ defmodule TzExtra.Compiler do
 
           Enum.map(start_unix_time..end_unix_time//step_in_seconds, &DateTime.from_unix!(&1))
         end
+
+        def advances_clock?(time_zone_id) when time_zone_id != nil do
+          case Tz.PeriodsProvider.periods(time_zone_id) do
+            {:error, :time_zone_not_found} ->
+              raise "invalid time zone #{time_zone_id}"
+
+            {:ok, [{utc_secs, _, _, nil} | _]} ->
+              hardcoded_dst_future_periods? =
+                DateTime.from_gregorian_seconds(utc_secs).year > Tz.PeriodsProvider.compiled_at().year + 20
+
+              hardcoded_dst_future_periods?
+
+            {:ok, [{_, _, _, _} | _]} ->
+              true
+          end
+        end
       end,
       for %{code: country_code} <- countries do
         {:ok, time_zones_for_country} =

test/tz_extra_test.exs

@@ -62,6 +62,14 @@ defmodule TzExtraTest do
     assert TzExtra.iana_version() == Tz.iana_version()
   end
 
+  test "advances_clock?/1" do
+    assert TzExtra.advances_clock?("Europe/Brussels")
+    assert TzExtra.advances_clock?("Africa/Casablanca")
+
+    refute TzExtra.advances_clock?("Asia/Manila")
+    refute TzExtra.advances_clock?("Asia/Tokyo")
+  end
+
   test "time_zone_id_exists?/1" do
     assert TzExtra.time_zone_id_exists?("Europe/Brussels")
     assert TzExtra.time_zone_id_exists?("Europe/Amsterdam")