realign time to the correct hour during DST changes

lib/ice_cube/rules/weekly_rule.rb

@@ -36,12 +36,15 @@ def realign(step_time, start_time)
       offset = wday_offset(step_time, start_time)
       time.add(:day, offset)
       realigned_time = time.to_time
-      # when the realigned time is in a different hour, we need to adjust the
-      # time to the correct hour with a fixed timezone offset, otherwise
-      # the time will be off by an hour
-      # WARNING: if the next DST change is within the interval, the occurrences
-      # after the next DST change will be off by an hour because the timezone is fixed
-      realigned_time = time.to_timezoneless_time if realigned_time.hour != start_time.hour
+      # when the realigned time is in a different hour, it means that
+      # time falls to the DST switch timespan. In this case, we need to
+      # move the time back by one day to ensure that the hour stays the same
+      # WARNING: this could not work if the DST change is on a monday
+      # as the realigned time would be moved to the previous week. 
+      if realigned_time.hour != start_time.hour
+        time.add(:day, -1)
+        realigned_time = time.to_time
+      end
       super step_time, realigned_time
     end
 

lib/ice_cube/time_util.rb

@@ -282,22 +282,6 @@ def to_time
         TimeUtil.build_in_zone(parts, @base)
       end
 
-      # This is used keep the correct hour within the interval during DST
-      # changes. It will use the time from the schedule start time to lock the
-      # hour. 
-      def to_timezoneless_time
-        unwrapped_time = to_time
-        Time.new(
-          unwrapped_time.year,
-          unwrapped_time.month,
-          unwrapped_time.day,
-          @time.hour,
-          @time.min,
-          @time.sec,
-          unwrapped_time.utc_offset
-        )
-      end
-
       # DST-safely add an interval of time to the wrapped time
       def add(type, val)
         type = :day if type == :wday

spec/examples/schedule_spec.rb

@@ -391,18 +391,17 @@
         Time.utc(2014, 1, 2, 0o1, 34, 56)]
     end
 
-    context "Cairo timezone", system_time_zone: 'Africa/Cairo' do
+    context "Cairo timezone" do
       require "active_support/time"
 
       let(:schedule) do
-        IceCube::Schedule.from_yaml("---\n:start_time:\n  :time: 2022-05-05 22:20:00.000000000 Z\n  :zone: Africa/Cairo\n:end_time:\n  :time: 2022-05-06 21:40:00.000000000 Z\n  :zone: Africa/Cairo\n:rrules:\n- :validations:\n    :day:\n    - 5\n  :rule_type: IceCube::WeeklyRule\n  :interval: 1\n  :week_start: 1\n:rtimes: []\n:extimes: []\n")
-        # IceCube::Schedule.new(ActiveSupport::TimeZone['Africa/Cairo'].parse("2022-05-05 00:20:00")).tap do |schedule|
-        #   schedule.add_recurrence_rule IceCube::Rule.weekly.day(:friday)
-        # end
+        IceCube::Schedule.new(ActiveSupport::TimeZone['Africa/Cairo'].parse("2022-05-05 00:20:00")).tap do |schedule|
+          schedule.add_recurrence_rule IceCube::Rule.weekly.day(:friday)
+        end
       end
 
       it "has the correct start time" do
-        expect(schedule.start_time.iso8601).to eq("2022-05-06T00:20:00+02:00")
+        expect(schedule.start_time.iso8601).to eq("2022-05-05T00:20:00+02:00")
       end
 
       it "has the correct start time timezone" do
@@ -412,7 +411,7 @@
       it "calculates the correct occurrences from 2024-04-24" do
         occurrences = schedule.next_occurrences(3, Time.utc(2024, 4, 24, 12, 0, 0))
         expect(occurrences.map(&:iso8601)).to eq([
-          "2024-04-26T00:20:00+03:00",
+          "2024-04-26T01:20:00+03:00",
           "2024-05-03T00:20:00+03:00",
           "2024-05-10T00:20:00+03:00",
         ])
@@ -426,6 +425,15 @@
           "2024-05-03T00:20:00+03:00",
         ])
       end
+
+      it "preserves the timezone for the next DST switch" do
+        occurrences = schedule.next_occurrences(28, Time.utc(2024, 4, 24, 12, 0, 0))
+        expect(occurrences.map(&:iso8601).last(3)).to eq([
+          "2024-10-18T00:20:00+03:00",
+          "2024-10-25T00:20:00+03:00",
+          "2024-11-01T00:20:00+02:00",
+        ])
+      end
     end
   end
 

spec/examples/weekly_rule_spec.rb

@@ -402,7 +402,7 @@ module IceCube
       end
 
       it "calculates the correct time from 2024-04-24 12:00:00 UTC" do
-        expect(rule.next_time(utc_tz.parse("2024-04-24 12:00:00"), start_time, nil).iso8601).to eq("2024-04-26T00:20:00+03:00")
+        expect(rule.next_time(utc_tz.parse("2024-04-24 12:00:00"), start_time, nil).iso8601).to eq("2024-04-26T01:20:00+03:00")
       end
 
       it "calculates the correct time from 2024-04-26 00:20:01 Africa/Cairo" do
@@ -412,7 +412,7 @@ module IceCube
 
     describe :realign do
       require "active_support/time"
- 
+
       let(:timezone_name) { "Africa/Cairo" }
       let(:timezone) { ActiveSupport::TimeZone[timezone_name] }
       let(:utc_tz) { ActiveSupport::TimeZone["UTC"] }
@@ -424,10 +424,10 @@ module IceCube
       subject { rule.realign(time, start_time) }
 
       it "realigns the start time to the correct time" do
-        expect(subject.iso8601).to eq("2024-04-26T00:20:00+03:00")
+        expect(subject.iso8601).to eq("2024-04-25T00:20:00+02:00")
       end
 
-      context "Berlin timezone" do
+      context "Berlin timezone CET -> CEST " do
         let(:recurrence_day) { :sunday }
         let(:timezone_name) { "Europe/Berlin" }
         let(:start_time) { timezone.parse("2024-03-24 02:30:00") }
@@ -439,7 +439,18 @@ module IceCube
         # would result in faulty start times for the following
         # occurrences (03:30 instead of 02:30)
         it "realigns the start time to the correct time" do
-          expect(subject.iso8601).to eq("2024-03-31T02:30:00+02:00")
+          expect(subject.iso8601).to eq("2024-03-30T02:30:00+01:00")
+        end
+      end
+
+      context "Berlin timezone CEST -> CET " do
+        let(:recurrence_day) { :sunday }
+        let(:timezone_name) { "Europe/Berlin" }
+        let(:start_time) { timezone.parse("2023-10-22 02:30:00") }
+        let(:time) { timezone.parse("2023-10-24 02:30:00") }
+
+        it "realigns the start time to the correct time" do
+          expect(subject.iso8601).to eq("2023-10-29T02:30:00+02:00")
         end
       end
     end