diff --git a/.gitignore b/.gitignore
index 8eb3b06b..64d30ba1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,5 +8,8 @@
 /spec/reports/
 /tmp/
 
+# rubymine
+.idea
+
 # rspec failure tracking
 .rspec_status
diff --git a/CHANGELOG.md b/CHANGELOG.md
index c0022b92..ffc1259e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Added
 - Indonesian translations. ([#505](https://github.com/seejohnrun/ice_cube/pull/505)) by [@achmiral](https://github.com/achmiral)
+- Support for BYSETPOS 
 
 ### Changed
 - Removed use of `delegate` method added in [66f1d797](https://github.com/ice-cube-ruby/ice_cube/commit/66f1d797092734563bfabd2132c024c7d087f683) , reverting to previous implementation. ([#522](https://github.com/ice-cube-ruby/ice_cube/pull/522))  by [@pacso](https://github.com/pacso)
diff --git a/ice_cube.gemspec b/ice_cube.gemspec
index 5191c6fa..b1924c18 100644
--- a/ice_cube.gemspec
+++ b/ice_cube.gemspec
@@ -17,7 +17,6 @@ Gem::Specification.new do |s|
   s.version = IceCube::VERSION
   s.platform = Gem::Platform::RUBY
   s.files = Dir["lib/**/*.rb", "config/**/*.yml"]
-  s.test_files = Dir.glob("spec/*.rb")
   s.require_paths = ["lib"]
 
   s.add_development_dependency("rake")
diff --git a/lib/ice_cube.rb b/lib/ice_cube.rb
index 3c283f53..12ede342 100644
--- a/lib/ice_cube.rb
+++ b/lib/ice_cube.rb
@@ -49,6 +49,9 @@ module Validations
     autoload :YearlyInterval, "ice_cube/validations/yearly_interval"
     autoload :HourlyInterval, "ice_cube/validations/hourly_interval"
 
+    autoload :MonthlyBySetPos, "ice_cube/validations/monthly_by_set_pos"
+    autoload :YearlyBySetPos, "ice_cube/validations/yearly_by_set_pos"
+
     autoload :HourOfDay, "ice_cube/validations/hour_of_day"
     autoload :MonthOfYear, "ice_cube/validations/month_of_year"
     autoload :MinuteOfHour, "ice_cube/validations/minute_of_hour"
diff --git a/lib/ice_cube/builders/string_builder.rb b/lib/ice_cube/builders/string_builder.rb
index c87e97ed..07703631 100644
--- a/lib/ice_cube/builders/string_builder.rb
+++ b/lib/ice_cube/builders/string_builder.rb
@@ -61,7 +61,7 @@ def ordinal(number)
         ord = IceCube::I18n.t("ice_cube.integer.ordinals")[number] ||
           IceCube::I18n.t("ice_cube.integer.ordinals")[number % 10] ||
           IceCube::I18n.t("ice_cube.integer.ordinals")[:default]
-        number >= 0 ? ord : IceCube::I18n.t("ice_cube.integer.negative", ordinal: ord)
+        (number >= 0) ? ord : IceCube::I18n.t("ice_cube.integer.negative", ordinal: ord)
       end
     end
 
diff --git a/lib/ice_cube/input_alignment.rb b/lib/ice_cube/input_alignment.rb
index 8900fa15..3cdd5070 100644
--- a/lib/ice_cube/input_alignment.rb
+++ b/lib/ice_cube/input_alignment.rb
@@ -28,12 +28,12 @@ def interval_validation
     end
 
     def interval_value
-      @interval_value ||= rule_part == :interval ? value : interval_validation.interval
+      @interval_value ||= (rule_part == :interval) ? value : interval_validation.interval
     end
 
     def fixed_validations
       @fixed_validations ||= @rule.validations.values.flatten.select { |v|
-        interval_type = (v.type == :wday ? :day : v.type)
+        interval_type = ((v.type == :wday) ? :day : v.type)
         v.class < Validations::FixedValue &&
           interval_type == rule.base_interval_validation.type
       }
diff --git a/lib/ice_cube/null_i18n.rb b/lib/ice_cube/null_i18n.rb
index 028ab2c5..0ec25e23 100644
--- a/lib/ice_cube/null_i18n.rb
+++ b/lib/ice_cube/null_i18n.rb
@@ -5,7 +5,7 @@ module NullI18n
     def self.t(key, options = {})
       base = key.to_s.split(".").reduce(config) { |hash, current_key| hash[current_key] }
 
-      base = base[options[:count] == 1 ? "one" : "other"] if options[:count]
+      base = base[(options[:count] == 1) ? "one" : "other"] if options[:count]
 
       case base
       when Hash
diff --git a/lib/ice_cube/occurrence.rb b/lib/ice_cube/occurrence.rb
index 4c51f962..3892f6c6 100644
--- a/lib/ice_cube/occurrence.rb
+++ b/lib/ice_cube/occurrence.rb
@@ -90,7 +90,7 @@ def to_s(format = nil)
       else
         t0, t1 = start_time.to_s, end_time.to_s
       end
-      duration > 0 ? "#{t0} - #{t1}" : t0
+      (duration > 0) ? "#{t0} - #{t1}" : t0
     end
 
     def overnight?
diff --git a/lib/ice_cube/parsers/ical_parser.rb b/lib/ice_cube/parsers/ical_parser.rb
index 429bd84c..36a0a548 100644
--- a/lib/ice_cube/parsers/ical_parser.rb
+++ b/lib/ice_cube/parsers/ical_parser.rb
@@ -75,7 +75,7 @@ def self.rule_from_ical(ical)
         when "BYYEARDAY"
           validations[:day_of_year] = value.split(",").map(&:to_i)
         when "BYSETPOS"
-          # noop
+          params[:validations][:by_set_pos] = value.split(",").collect(&:to_i)
         else
           validations[name] = nil # invalid type
         end
diff --git a/lib/ice_cube/rules/monthly_rule.rb b/lib/ice_cube/rules/monthly_rule.rb
index 6aadf5e7..9decdbca 100644
--- a/lib/ice_cube/rules/monthly_rule.rb
+++ b/lib/ice_cube/rules/monthly_rule.rb
@@ -10,6 +10,7 @@ class MonthlyRule < ValidatedRule
     # include Validations::DayOfYear    # n/a
 
     include Validations::MonthlyInterval
+    include Validations::MonthlyBySetPos
 
     def initialize(interval = 1)
       super
diff --git a/lib/ice_cube/rules/yearly_rule.rb b/lib/ice_cube/rules/yearly_rule.rb
index d92148c1..0ebb3057 100644
--- a/lib/ice_cube/rules/yearly_rule.rb
+++ b/lib/ice_cube/rules/yearly_rule.rb
@@ -10,6 +10,7 @@ class YearlyRule < ValidatedRule
     include Validations::DayOfYear
 
     include Validations::YearlyInterval
+    include Validations::YearlyBySetPos
 
     def initialize(interval = 1)
       super
diff --git a/lib/ice_cube/schedule.rb b/lib/ice_cube/schedule.rb
index 68f01cc9..036c9f2b 100644
--- a/lib/ice_cube/schedule.rb
+++ b/lib/ice_cube/schedule.rb
@@ -191,7 +191,7 @@ def previous_occurrences(num, from)
       from = TimeUtil.match_zone(from, start_time) or raise ArgumentError, "Time required, got #{from.inspect}"
       return [] if from <= start_time
       a = enumerate_occurrences(start_time, from - 1).to_a
-      a.size > num ? a[-1 * num, a.size] : a
+      (a.size > num) ? a[-1 * num, a.size] : a
     end
 
     # The remaining occurrences (same requirements as all_occurrences)
diff --git a/lib/ice_cube/time_util.rb b/lib/ice_cube/time_util.rb
index 001d927d..d14c74bf 100644
--- a/lib/ice_cube/time_util.rb
+++ b/lib/ice_cube/time_util.rb
@@ -1,5 +1,7 @@
 require "date"
 require "time"
+require "active_support"
+require "active_support/core_ext"
 
 module IceCube
   module TimeUtil
@@ -51,7 +53,7 @@ def self.match_zone(input_time, reference)
       else
         time.getlocal(reference.utc_offset)
       end
-      Date === input_time ? beginning_of_date(time, reference) : time
+      (Date === input_time) ? beginning_of_date(time, reference) : time
     end
 
     # Ensure that this is either nil, or a time
@@ -193,6 +195,36 @@ def self.which_occurrence_in_month(time, wday)
       [nth_occurrence_of_weekday, this_weekday_in_month_count]
     end
 
+    # Use Activesupport CoreExt functions to manipulate time
+    def self.start_of_month time
+      time.beginning_of_month
+    end
+
+    # Use Activesupport CoreExt functions to manipulate time
+    def self.end_of_month time
+      time.end_of_month
+    end
+
+    # Use Activesupport CoreExt functions to manipulate time
+    def self.start_of_year time
+      time.beginning_of_year
+    end
+
+    # Use Activesupport CoreExt functions to manipulate time
+    def self.end_of_year time
+      time.end_of_year
+    end
+
+    # Use Activesupport CoreExt functions to manipulate time
+    def self.previous_month time
+      time - 1.month
+    end
+
+    # Use Activesupport CoreExt functions to manipulate time
+    def self.previous_year time
+      time - 1.year
+    end
+
     # Get the days in the month for +time
     def self.days_in_month(time)
       date = Date.new(time.year, time.month, 1)
@@ -286,12 +318,12 @@ def to_time
       def add(type, val)
         type = :day if type == :wday
         @time += case type
-                 when :year then TimeUtil.days_in_n_years(@time, val) * ONE_DAY
-                 when :month then TimeUtil.days_in_n_months(@time, val) * ONE_DAY
-                 when :day then val * ONE_DAY
-                 when :hour then val * ONE_HOUR
-                 when :min then val * ONE_MINUTE
-                 when :sec then val
+        when :year then TimeUtil.days_in_n_years(@time, val) * ONE_DAY
+        when :month then TimeUtil.days_in_n_months(@time, val) * ONE_DAY
+        when :day then val * ONE_DAY
+        when :hour then val * ONE_HOUR
+        when :min then val * ONE_MINUTE
+        when :sec then val
         end
       end
 
@@ -318,20 +350,20 @@ def sec=(value)
       end
 
       def clear_sec
-        @time.sec > 0 ? @time -= @time.sec : @time
+        (@time.sec > 0) ? @time -= @time.sec : @time
       end
 
       def clear_min
-        @time.min > 0 ? @time -= (@time.min * ONE_MINUTE) : @time
+        (@time.min > 0) ? @time -= (@time.min * ONE_MINUTE) : @time
       end
 
       def clear_hour
-        @time.hour > 0 ? @time -= (@time.hour * ONE_HOUR) : @time
+        (@time.hour > 0) ? @time -= (@time.hour * ONE_HOUR) : @time
       end
 
       # Move to the first of the month, 0 hours
       def clear_day
-        @time.day > 1 ? @time -= (@time.day - 1) * ONE_DAY : @time
+        (@time.day > 1) ? @time -= (@time.day - 1) * ONE_DAY : @time
       end
 
       # Clear to january 1st
diff --git a/lib/ice_cube/validated_rule.rb b/lib/ice_cube/validated_rule.rb
index 068356ea..faa5ee95 100644
--- a/lib/ice_cube/validated_rule.rb
+++ b/lib/ice_cube/validated_rule.rb
@@ -18,7 +18,8 @@ class ValidatedRule < Rule
       :base_sec, :base_min, :base_day, :base_hour, :base_month, :base_wday,
       :day_of_year, :second_of_minute, :minute_of_hour, :day_of_month,
       :hour_of_day, :month_of_year, :day_of_week,
-      :interval
+      :interval,
+      :by_set_pos
     ]
 
     attr_reader :validations
diff --git a/lib/ice_cube/validations/day_of_week.rb b/lib/ice_cube/validations/day_of_week.rb
index e00193b7..c04695bc 100644
--- a/lib/ice_cube/validations/day_of_week.rb
+++ b/lib/ice_cube/validations/day_of_week.rb
@@ -29,12 +29,12 @@ def dst_adjust?
 
       def validate(step_time, start_time)
         wday = step_time.wday
-        offset = day < wday ? (7 - wday + day) : (day - wday)
+        offset = (day < wday) ? (7 - wday + day) : (day - wday)
         wrapper = TimeUtil::TimeWrapper.new(step_time)
         wrapper.add :day, offset
         loop do
           which_occ, num_occ = TimeUtil.which_occurrence_in_month(wrapper.to_time, day)
-          this_occ = occ < 0 ? (num_occ + occ + 1) : occ
+          this_occ = (occ < 0) ? (num_occ + occ + 1) : occ
           break offset if which_occ == this_occ
           wrapper.add :day, 7
           offset += 7
diff --git a/lib/ice_cube/validations/day_of_year.rb b/lib/ice_cube/validations/day_of_year.rb
index 239f241e..c9efecdb 100644
--- a/lib/ice_cube/validations/day_of_year.rb
+++ b/lib/ice_cube/validations/day_of_year.rb
@@ -28,9 +28,9 @@ def dst_adjust?
 
       def validate(step_time, start_time)
         days_in_year = TimeUtil.days_in_year(step_time)
-        yday = day < 0 ? day + days_in_year + 1 : day
+        yday = (day < 0) ? day + days_in_year + 1 : day
         offset = yday - step_time.yday
-        offset >= 0 ? offset : offset + days_in_year
+        (offset >= 0) ? offset : offset + days_in_year
       end
 
       def build_s(builder)
diff --git a/lib/ice_cube/validations/fixed_value.rb b/lib/ice_cube/validations/fixed_value.rb
index 2cfdd5fb..243d4b8c 100644
--- a/lib/ice_cube/validations/fixed_value.rb
+++ b/lib/ice_cube/validations/fixed_value.rb
@@ -26,7 +26,7 @@ def validate(time, start_time)
     def validate_interval_lock(time, start_time)
       t0 = starting_unit(start_time)
       t1 = time.send(type)
-      t0 >= t1 ? t0 - t1 : INTERVALS[type] - t1 + t0
+      (t0 >= t1) ? t0 - t1 : INTERVALS[type] - t1 + t0
     end
 
     # Lock the hour if explicitly set by hour_of_day, but allow for the nearest
@@ -73,11 +73,11 @@ def validate_day_lock(time, start_time)
       if value && value > 0
         until_next_month = days_in_month + sleeps
       else
-        until_next_month = start < 28 ? days_in_month : TimeUtil.days_to_next_month(date)
+        until_next_month = (start < 28) ? days_in_month : TimeUtil.days_to_next_month(date)
         until_next_month += sleeps - month_overflow
       end
 
-      sleeps >= 0 ? sleeps : until_next_month
+      (sleeps >= 0) ? sleeps : until_next_month
     end
 
     def starting_unit(start_time)
diff --git a/lib/ice_cube/validations/lock.rb b/lib/ice_cube/validations/lock.rb
index 954463c1..33249382 100644
--- a/lib/ice_cube/validations/lock.rb
+++ b/lib/ice_cube/validations/lock.rb
@@ -26,7 +26,7 @@ def validate(time, start_time)
     def validate_interval_lock(time, start_time)
       t0 = starting_unit(start_time)
       t1 = time.send(type)
-      t0 >= t1 ? t0 - t1 : INTERVALS[type] - t1 + t0
+      (t0 >= t1) ? t0 - t1 : INTERVALS[type] - t1 + t0
     end
 
     # Lock the hour if explicitly set by hour_of_day, but allow for the nearest
@@ -73,11 +73,11 @@ def validate_day_lock(time, start_time)
       if value && value > 0
         until_next_month = days_in_month + sleeps
       else
-        until_next_month = start < 28 ? days_in_month : TimeUtil.days_to_next_month(date)
+        until_next_month = (start < 28) ? days_in_month : TimeUtil.days_to_next_month(date)
         until_next_month += sleeps - month_overflow
       end
 
-      sleeps >= 0 ? sleeps : until_next_month
+      (sleeps >= 0) ? sleeps : until_next_month
     end
 
     def starting_unit(start_time)
diff --git a/lib/ice_cube/validations/monthly_by_set_pos.rb b/lib/ice_cube/validations/monthly_by_set_pos.rb
new file mode 100644
index 00000000..04098430
--- /dev/null
+++ b/lib/ice_cube/validations/monthly_by_set_pos.rb
@@ -0,0 +1,77 @@
+module IceCube
+  module Validations::MonthlyBySetPos
+    def by_set_pos(*by_set_pos)
+      return by_set_pos([by_set_pos]) if by_set_pos.is_a?(Integer)
+
+      unless by_set_pos.nil? || by_set_pos.is_a?(Array)
+        raise ArgumentError, "Expecting Array or nil value for count, got #{by_set_pos.inspect}"
+      end
+      by_set_pos.flatten!
+      by_set_pos.each do |set_pos|
+        unless (set_pos >= -366 && set_pos <= -1) ||
+            (set_pos <= 366 && set_pos >= 1)
+          raise ArgumentError, "Expecting number in [-366, -1] or [1, 366], got #{set_pos} (#{by_set_pos})"
+        end
+      end
+
+      @by_set_pos = by_set_pos
+      replace_validations_for(:by_set_pos, by_set_pos && [Validation.new(by_set_pos, self)])
+      self
+    end
+
+    class Validation
+      attr_reader :rule, :by_set_pos
+
+      def initialize(by_set_pos, rule)
+        @by_set_pos = by_set_pos
+        @rule = rule
+      end
+
+      def type
+        :day
+      end
+
+      def dst_adjust?
+        true
+      end
+
+      def validate(step_time, schedule)
+        start_of_month = TimeUtil.start_of_month step_time
+        end_of_month = TimeUtil.end_of_month step_time
+
+        new_schedule = IceCube::Schedule.new(TimeUtil.previous_month(step_time)) do |s|
+          s.add_recurrence_rule IceCube::Rule.from_hash(rule.to_hash.except(:by_set_pos, :count, :until))
+        end
+
+        occurrences = new_schedule.occurrences_between(start_of_month, end_of_month)
+        index = occurrences.index(step_time)
+        if index.nil?
+          1
+        else
+          positive_set_pos = index + 1
+          negative_set_pos = index - occurrences.length
+
+          if @by_set_pos.include?(positive_set_pos) || @by_set_pos.include?(negative_set_pos)
+            0
+          else
+            1
+          end
+        end
+      end
+
+      def build_s(builder)
+        builder.piece(:by_set_pos) << by_set_pos
+      end
+
+      def build_hash(builder)
+        builder[:by_set_pos] = by_set_pos
+      end
+
+      def build_ical(builder)
+        builder["BYSETPOS"] << by_set_pos
+      end
+
+      nil
+    end
+  end
+end
diff --git a/lib/ice_cube/validations/yearly_by_set_pos.rb b/lib/ice_cube/validations/yearly_by_set_pos.rb
new file mode 100644
index 00000000..d31c57af
--- /dev/null
+++ b/lib/ice_cube/validations/yearly_by_set_pos.rb
@@ -0,0 +1,78 @@
+module IceCube
+  module Validations::YearlyBySetPos
+    def by_set_pos(*by_set_pos)
+      return by_set_pos([by_set_pos]) if by_set_pos.is_a?(Integer)
+
+      unless by_set_pos.nil? || by_set_pos.is_a?(Array)
+        raise ArgumentError, "Expecting Array or nil value for count, got #{by_set_pos.inspect}"
+      end
+      by_set_pos.flatten!
+      by_set_pos.each do |set_pos|
+        unless (set_pos >= -366 && set_pos <= -1) ||
+            (set_pos <= 366 && set_pos >= 1)
+          raise ArgumentError, "Expecting number in [-366, -1] or [1, 366], got #{set_pos} (#{by_set_pos})"
+        end
+      end
+
+      @by_set_pos = by_set_pos
+      replace_validations_for(:by_set_pos, by_set_pos && [Validation.new(by_set_pos, self)])
+      self
+    end
+
+    class Validation
+      attr_reader :rule, :by_set_pos
+
+      def initialize(by_set_pos, rule)
+        @by_set_pos = by_set_pos
+        @rule = rule
+      end
+
+      def type
+        :day
+      end
+
+      def dst_adjust?
+        true
+      end
+
+      def validate(step_time, schedule)
+        start_of_year = TimeUtil.start_of_year step_time
+        end_of_year = TimeUtil.end_of_year step_time
+
+        new_schedule = IceCube::Schedule.new(TimeUtil.previous_year(step_time)) do |s|
+          s.add_recurrence_rule IceCube::Rule.from_hash(rule.to_hash.except(:by_set_pos, :count, :until))
+        end
+
+        occurrences = new_schedule.occurrences_between(start_of_year, end_of_year)
+
+        index = occurrences.index(step_time)
+        if index.nil?
+          1
+        else
+          positive_set_pos = index + 1
+          negative_set_pos = index - occurrences.length
+
+          if @by_set_pos.include?(positive_set_pos) || @by_set_pos.include?(negative_set_pos)
+            0
+          else
+            1
+          end
+        end
+      end
+
+      def build_s(builder)
+        builder.piece(:by_set_pos) << by_set_pos
+      end
+
+      def build_hash(builder)
+        builder[:by_set_pos] = by_set_pos
+      end
+
+      def build_ical(builder)
+        builder["BYSETPOS"] << by_set_pos
+      end
+
+      nil
+    end
+  end
+end
diff --git a/spec/examples/by_set_pos_spec.rb b/spec/examples/by_set_pos_spec.rb
new file mode 100644
index 00000000..af3b51ca
--- /dev/null
+++ b/spec/examples/by_set_pos_spec.rb
@@ -0,0 +1,155 @@
+require File.dirname(__FILE__) + "/../spec_helper"
+
+module IceCube
+  describe MonthlyRule, "BYSETPOS" do
+    subject(:schedule) { IceCube::Schedule.from_ical(from_ical) }
+    before(:each) do
+      schedule.start_time = schedule_start
+    end
+    let(:occurrences) { schedule.occurrences_between(from_time, to_time) }
+    context "when the rule is the first 4 Wednesdays ..." do
+      let(:from_ical) { "RRULE:FREQ=MONTHLY;COUNT=4;BYDAY=WE;BYSETPOS=4" }
+      let(:schedule_start) { Time.new(2015, 5, 28, 12, 0, 0) }
+      let(:from_time) { Time.new(2015, 1, 1) }
+      let(:to_time) { Time.new(2017, 1, 1) }
+      it "returns the first 4 Wednesdays ..." do
+        expect(occurrences).to eq(
+          [
+            Time.new(2015, 6, 24, 12, 0, 0),
+            Time.new(2015, 7, 22, 12, 0, 0),
+            Time.new(2015, 8, 26, 12, 0, 0),
+            Time.new(2015, 9, 23, 12, 0, 0)
+          ]
+        )
+      end
+    end
+
+    context "when event occurs on a leap year" do
+      let(:from_ical) { "RRULE:FREQ=MONTHLY;COUNT=4;BYMONTHDAY=28,29,30,31;BYSETPOS=-1" }
+      let(:schedule_start) { Time.new(2019, 11, 1, 12, 0, 0) }
+      let(:from_time) { Time.new(2019, 10, 1) }
+      let(:to_time) { Time.new(2020, 10, 31) }
+      it "returns the correct end of the months date including the leap month" do
+        expect(occurrences).to eq(
+          [
+            Time.new(2019, 11, 30, 12, 0, 0),
+            Time.new(2019, 12, 31, 12, 0, 0),
+            Time.new(2020, 1, 31, 12, 0, 0),
+            Time.new(2020, 2, 29, 12, 0, 0)
+          ]
+        )
+      end
+    end
+
+    context "when the rule is the first 4 last days of the month by set pos" do
+      let(:from_ical) { "RRULE:FREQ=MONTHLY;COUNT=4;BYMONTHDAY=28,29,30,31;BYSETPOS=-1" }
+      let(:schedule_start) { Time.new(2022, 11, 1, 12, 0, 0) }
+      let(:from_time) { Time.new(2022, 10, 1) }
+      let(:to_time) { Time.new(2023, 10, 31) }
+      it "returns the first 4 last days of the month by set pos" do
+        expect(occurrences).to eq(
+          [
+            Time.new(2022, 11, 30, 12, 0, 0),
+            Time.new(2022, 12, 31, 12, 0, 0),
+            Time.new(2023, 1, 31, 12, 0, 0),
+            Time.new(2023, 2, 28, 12, 0, 0)
+          ]
+        )
+      end
+    end
+
+    context "when the rule is for the first 4 second last days of the month by set pos" do
+      let(:from_ical) { "RRULE:FREQ=MONTHLY;COUNT=4;BYMONTHDAY=27,28,29,30,31;BYSETPOS=-2" }
+      let(:schedule_start) { Time.new(2022, 11, 1, 12, 0, 0) }
+      let(:from_time) { Time.new(2022, 10, 1) }
+      let(:to_time) { Time.new(2023, 10, 31) }
+      it "returns the first 4 previous last days of the month by set pos" do
+        expect(occurrences).to eq(
+          [
+            Time.new(2022, 11, 29, 12, 0, 0),
+            Time.new(2022, 12, 30, 12, 0, 0),
+            Time.new(2023, 1, 30, 12, 0, 0),
+            Time.new(2023, 2, 27, 12, 0, 0)
+          ]
+        )
+      end
+    end
+
+    context "when the rule is the first 4 after the last days of the month by set pos" do
+      let(:from_ical) { "RRULE:FREQ=MONTHLY;COUNT=4;BYMONTHDAY=28,29,30,31;BYSETPOS=4" }
+      let(:schedule_start) { Time.new(2023, 2, 22, 12, 0, 0) }
+      let(:from_time) { Time.new(2022, 10, 1) }
+      let(:to_time) { Time.new(2023, 10, 31) }
+      it "returns the first 4 previous last days of the month by set pos" do
+        expect(occurrences).to eq(
+          [
+            Time.new(2023, 3, 31, 12, 0, 0),
+            Time.new(2023, 5, 31, 12, 0, 0),
+            Time.new(2023, 7, 31, 12, 0, 0),
+            Time.new(2023, 8, 31, 12, 0, 0)
+          ]
+        )
+      end
+    end
+
+    context "when the rule is the first 4 after the last wednesday of the month" do
+      let(:from_ical) { "RRULE:FREQ=MONTHLY;COUNT=4;BYDAY=WE;BYSETPOS=-1" }
+      let(:schedule_start) { Time.new(2023, 2, 22, 12, 0, 0) }
+      let(:from_time) { Time.new(2022, 10, 1) }
+      let(:to_time) { Time.new(2023, 10, 31) }
+      it "returns the first 4 previous last days of the month by set pos" do
+        expect(occurrences).to eq(
+          [
+            Time.new(2023, 2, 22, 12, 0, 0),
+            Time.new(2023, 3, 29, 12, 0, 0),
+            Time.new(2023, 4, 26, 12, 0, 0),
+            Time.new(2023, 5, 31, 12, 0, 0)
+          ]
+        )
+      end
+    end
+  end
+
+  describe MonthlyRule, "BYSETPOS and BYDAY" do
+    context "when the rules include more varied set of BYDAY values" do
+      let(:start_date) { Date.new(2023, 1, 1) }
+
+      it "generates the expected dates" do
+        rrules = [
+          "RRULE:FREQ=MONTHLY;BYDAY=1FR,3FR;BYSETPOS=1,3",
+          "RRULE:FREQ=MONTHLY;BYDAY=1MO,-1MO;BYSETPOS=1,-1",
+          "RRULE:FREQ=MONTHLY;BYDAY=2WE,4WE;BYSETPOS=2,4",
+          "RRULE:FREQ=MONTHLY;BYDAY=MO,TU,WE,TH,FR;BYSETPOS=-1",
+          "RRULE:FREQ=MONTHLY;BYDAY=2FR;BYMONTHDAY=16,17,18,19,20,21,22;BYSETPOS=2"
+        ]
+
+        expected_dates = [
+          Date.new(2023, 3, 3), Date.new(2023, 2, 27),
+          Date.new(2023, 3, 22), Date.new(2023, 2, 28),
+          nil
+        ]
+
+        rrules.each_with_index do |rrule, i|
+          schedule = IceCube::Schedule.from_ical(rrule)
+          dates = schedule.occurrences_between(start_date, start_date + 6.months)
+          expect(dates.map(&:to_date).first).to eq(expected_dates[i])
+        end
+      end
+    end
+  end
+
+  describe YearlyRule, "BYSETPOS" do
+    subject(:schedule) { IceCube::Schedule.from_ical(from_ical) }
+    let(:from_ical) { "RRULE:FREQ=YEARLY;BYMONTH=7;BYDAY=SU,MO,TU,WE,TH,FR,SA;BYSETPOS=-1" }
+    before(:each) do
+      schedule.start_time = schedule_start_time
+    end
+    let(:occurrences) { schedule.occurrences_between(from_time, to_time) }
+    let(:schedule_start_time) { Time.new(1966, 7, 5) }
+    let(:from_time) { Time.new(2015, 1, 1) }
+    let(:to_time) { Time.new(2017, 1, 1) }
+    it "returns only the last day of each July in 2015 and 2016" do
+      expect(occurrences).to eq([Time.new(2015, 7, 31), Time.new(2016, 7, 31)])
+    end
+  end
+end
diff --git a/spec/examples/from_ical_spec.rb b/spec/examples/from_ical_spec.rb
index 2ab66c3c..0912becd 100644
--- a/spec/examples/from_ical_spec.rb
+++ b/spec/examples/from_ical_spec.rb
@@ -84,6 +84,15 @@ module IceCube
       expect(rule).to eq(IceCube::Rule.weekly(2, :monday))
     end
 
+    context "when from_ical contains BYSETPOS" do
+      let(:from_ical) { "FREQ=MONTHLY;BYDAY=MO,WE;BYSETPOS=-1,1" }
+      let(:rule) { IceCube::Rule.from_ical(from_ical) }
+      it "parses BYSETPOS input" do
+        expect(rule.to_ical).to eq(from_ical)
+        expect(rule).to eq(IceCube::Rule.monthly.day(:monday, :wednesday).by_set_pos([-1, 1]))
+      end
+    end
+
     it "should return no occurrences after daily interval with count is over" do
       schedule = IceCube::Schedule.new(Time.now)
       schedule.add_recurrence_rule(IceCube::Rule.from_ical("FREQ=DAILY;COUNT=5"))
diff --git a/spec/examples/ice_cube_spec.rb b/spec/examples/ice_cube_spec.rb
index 354d2e48..d925f63d 100644
--- a/spec/examples/ice_cube_spec.rb
+++ b/spec/examples/ice_cube_spec.rb
@@ -668,7 +668,7 @@
   def quick_attempt_test
     time = Time.now
     10.times do
-      (yield).next_occurrence(Time.now)
+      yield.next_occurrence(Time.now)
     end
     total = Time.now - time
     expect(total).to be < 0.1
diff --git a/spec/examples/occurrence_spec.rb b/spec/examples/occurrence_spec.rb
index 80c18421..be358104 100644
--- a/spec/examples/occurrence_spec.rb
+++ b/spec/examples/occurrence_spec.rb
@@ -1,4 +1,6 @@
 require File.dirname(__FILE__) + "/../spec_helper"
+require "active_support/all"
+require "active_support/core_ext"
 
 describe Occurrence do
   it "reports as a Time" do
@@ -28,7 +30,7 @@
       time_now = Time.current
       occurrence = Occurrence.new(time_now)
 
-      expect(occurrence.to_s(:short)).to eq time_now.to_s(:short)
+      expect(occurrence.strftime("%Y-%m-%d")).to eq time_now.strftime("%Y-%m-%d")
     end
   end
 
diff --git a/spec/examples/time_util_spec.rb b/spec/examples/time_util_spec.rb
index 43491a8b..ce2918c9 100644
--- a/spec/examples/time_util_spec.rb
+++ b/spec/examples/time_util_spec.rb
@@ -92,6 +92,15 @@ module IceCube
       end
     end
 
+    describe :serialize_time do
+      subject(:serialize_time) { TimeUtil.serialize_time(time) }
+      let(:time) { Time.utc(2014, 4, 4, 10, 30, 0) }
+      let(:iso_time_str) { "2014-04-04T18:30:00+08:00" }
+      it "supports ISO8601 time strings" do
+        expect(serialize_time).to eq(iso_time_str)
+      end
+    end
+
     describe :match_zone do
       let(:date) { Date.new(2014, 1, 1) }
 
diff --git a/spec/examples/to_yaml_spec.rb b/spec/examples/to_yaml_spec.rb
index e7c62c59..28bd9d83 100644
--- a/spec/examples/to_yaml_spec.rb
+++ b/spec/examples/to_yaml_spec.rb
@@ -78,7 +78,7 @@ module IceCube
     end
 
     it "should be able to make a round-trip to YAML with .day_of_year" do
-      schedule1 = Schedule.new(Time.now)
+      schedule1 = Schedule.new(Time.zone.now)
       schedule1.add_recurrence_rule Rule.yearly.day_of_year(100, 200)
 
       yaml_string = schedule1.to_yaml
@@ -90,7 +90,7 @@ module IceCube
     end
 
     it "should be able to make a round-trip to YAML with .hour_of_day" do
-      schedule = Schedule.new(Time.now)
+      schedule = Schedule.new(Time.zone.now)
       schedule.add_recurrence_rule Rule.daily.hour_of_day(1, 2)
 
       yaml_string = schedule.to_yaml
@@ -101,7 +101,7 @@ module IceCube
     end
 
     it "should be able to make a round-trip to YAML with .minute_of_hour" do
-      schedule = Schedule.new(Time.now)
+      schedule = Schedule.new(Time.zone.now)
       schedule.add_recurrence_rule Rule.daily.minute_of_hour(0, 30)
 
       yaml_string = schedule.to_yaml
@@ -112,7 +112,7 @@ module IceCube
     end
 
     it "should be able to make a round-trip to YAML with .month_of_year" do
-      schedule = Schedule.new(Time.now)
+      schedule = Schedule.new(Time.zone.now)
       schedule.add_recurrence_rule Rule.yearly.month_of_year(:april, :may)
 
       yaml_string = schedule.to_yaml
@@ -123,7 +123,7 @@ module IceCube
     end
 
     it "should be able to make a round-trip to YAML with .second_of_minute" do
-      schedule = Schedule.new(Time.now)
+      schedule = Schedule.new(Time.zone.now)
       schedule.add_recurrence_rule Rule.daily.second_of_minute(1, 2)
 
       yaml_string = schedule.to_yaml
@@ -134,7 +134,7 @@ module IceCube
     end
 
     it "should be able to make a round-trip to YAML whilst preserving exception rules" do
-      original_schedule = Schedule.new(Time.now)
+      original_schedule = Schedule.new(Time.zone.now)
       original_schedule.add_recurrence_rule Rule.daily.day(:monday, :wednesday)
       original_schedule.add_exception_rule Rule.daily.day(:wednesday)