fix: Daylight saving

lib/cocktail/util.ex

@@ -19,10 +19,22 @@ defmodule Cocktail.Util do
   def shift_time(datetime, opts) do
     datetime
     |> Timex.shift(opts)
+    |> shift_dst(datetime)
     |> no_ms()
   end
 
   def no_ms(time) do
     Map.put(time, :microsecond, {0, 0})
   end
+
+  # In case of datetime we may expect the same timezone hour
+  # For example after daylight saving 10h MUST still 10h the next day.
+  # This behaviour could only happen on datetime with timezone (that include `std_offset`)
+  defp shift_dst(time, datetime) do
+    if offset = Map.get(datetime, :std_offset) do
+      Timex.shift(time, seconds: offset - time.std_offset)
+    else
+      time
+    end
+  end
 end

test/cocktail/daily_test.exs

@@ -163,4 +163,34 @@ defmodule Cocktail.DailyTest do
            |> Cocktail.Schedule.occurrences(~N[2015-01-24 18:30:00])
            |> Enum.take(100) == [~N[2015-01-24 18:30:00]]
   end
+
+  test "generating occurrences with spring forward transition" do
+    schedule =
+      ~Y[2022-03-12 18:00:00 America/Los_Angeles]
+      |> Schedule.new()
+      |> Schedule.add_recurrence_rule(:daily)
+
+    times = schedule |> Schedule.occurrences() |> Enum.take(3)
+
+    assert times == [
+             ~Y[2022-03-12 18:00:00 America/Los_Angeles],
+             ~Y[2022-03-13 18:00:00 America/Los_Angeles],
+             ~Y[2022-03-14 18:00:00 America/Los_Angeles]
+           ]
+  end
+
+  test "generating occurrences with fall back transition" do
+    schedule =
+      ~Y[2022-11-05 18:00:00 America/Los_Angeles]
+      |> Schedule.new()
+      |> Schedule.add_recurrence_rule(:daily)
+
+    times = schedule |> Schedule.occurrences() |> Enum.take(3)
+
+    assert times == [
+             ~Y[2022-11-05 18:00:00 America/Los_Angeles],
+             ~Y[2022-11-06 18:00:00 America/Los_Angeles],
+             ~Y[2022-11-07 18:00:00 America/Los_Angeles]
+           ]
+  end
 end

test/cocktail/weekly_test.exs

@@ -154,4 +154,19 @@ defmodule Cocktail.WeeklyTest do
              ~Y[2017-01-09 10:00:00 America/Los_Angeles]
            ]
   end
+
+  test "Weekly with dst transition" do
+    times =
+      ~Y[2022-03-12 06:00:00 America/Los_Angeles]
+      |> Cocktail.schedule()
+      |> Schedule.add_recurrence_rule(:weekly)
+      |> Cocktail.Schedule.occurrences()
+      |> Enum.take(3)
+
+    assert times == [
+             ~Y[2022-03-12 06:00:00 America/Los_Angeles],
+             ~Y[2022-03-19 06:00:00 America/Los_Angeles],
+             ~Y[2022-03-26 06:00:00 America/Los_Angeles]
+           ]
+  end
 end