diff --git a/.rubocop.yml b/.rubocop.yml
index 2f00bf1..c233e44 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -44,6 +44,9 @@ Style/AccessModifierDeclarations:
 Style/AsciiComments:
   Enabled: false
 
+Style/BlockDelimiters:
+  EnforcedStyle: "braces_for_chaining"
+
 Style/FrozenStringLiteralComment:
   Enabled: false
 
diff --git a/lib/time_iterator.rb b/lib/time_iterator.rb
index 5cc3d8d..a5e5330 100644
--- a/lib/time_iterator.rb
+++ b/lib/time_iterator.rb
@@ -1,32 +1,31 @@
 require "active_support"
-require "active_support/core_ext/integer/time"
-require_relative "time_iterator/core_ext/numeric"
-require_relative "time_iterator/core_ext/time"
+require "active_support/duration"
 
 # Time iteration.
 module TimeIterator
-  INFINITY = 1.0 / 0.0
-  PERIODS = {
-    second: :seconds,
-    minute: :minutes,
-    hour: :hours,
-    day: :days,
-    week: :weeks,
-    month: :months,
-    quarter: :quarters,
-    year: :years
-  }.freeze
-  ITERATE_BY = (PERIODS.keys + PERIODS.values).freeze
-
   class << self
-    def iterate(start, by:, every: 1)
-      by = by.to_sym
+    def iterate(start, by:, to: nil)
+      raise ArgumentError, "`by` must be an ActiveSupport::Duration" unless by.is_a?(ActiveSupport::Duration)
+
+      return to ? iterate_to(start, by: by, to: to) : iterate_endless(start, by: by)
+    end
 
-      raise ArgumentError, "Unknown period to iterate by: #{by}" unless ITERATE_BY.include?(by)
+    private def iterate_endless(start, by:)
+      Enumerator.new do |block|
+        time = start
+        loop do
+          block << time
+          time += by
+        end
+      end
+    end
 
+    private def iterate_to(start, by:, to:)
       Enumerator.new do |block|
-        (0..INFINITY).each do |num|
-          block << (start + (num.send(by) * every))
+        time = start
+        while time <= to
+          block << time
+          time += by
         end
       end
     end
diff --git a/lib/time_iterator/core_ext/numeric.rb b/lib/time_iterator/core_ext/numeric.rb
deleted file mode 100644
index 8739872..0000000
--- a/lib/time_iterator/core_ext/numeric.rb
+++ /dev/null
@@ -1,7 +0,0 @@
-# Inject 3.quarters
-class Numeric
-  def quarter
-    (3 * self).months
-  end
-  alias quarters quarter
-end
diff --git a/lib/time_iterator/core_ext/time.rb b/lib/time_iterator/core_ext/time.rb
deleted file mode 100644
index 0d61719..0000000
--- a/lib/time_iterator/core_ext/time.rb
+++ /dev/null
@@ -1,48 +0,0 @@
-# Inject convenience methods into Time.
-class Time
-  private def valid_period?(period)
-    raise ArgumentError, "Unknown time period: #{period}" unless TimeIterator::PERIODS.include?(period)
-  end
-
-  private def method_for_period(method, period)
-    period = period.to_sym
-    valid_period?(period)
-    send("#{method}_#{period}")
-  end
-
-  def beginning_of(period)
-    method_for_period(:beginning_of, period)
-  end
-
-  def end_of(period)
-    method_for_period(:end_of, period)
-  end
-
-  def days_in_month
-    ::Time.days_in_month( month, year )
-  end
-
-  def days_in_year
-    ::Time.days_in_year( year )
-  end
-
-  # Creates a Range from the Time to the end of today
-  # @return [Range]
-  def span_to_end_of_today
-    beginning_of_next_day = ::Time.current.tomorrow.beginning_of_day
-    (self...beginning_of_next_day)
-  end
-
-  # @return [String] the abbreviated day of the week
-  def dow
-    strftime("%a")
-  end
-
-  def iso_date
-    strftime("%Y-%m-%d")
-  end
-
-  def human_date
-    strftime("%a %b %-d %Y")
-  end
-end
diff --git a/spec/time_iterator/core_ext/numeric_spec.rb b/spec/time_iterator/core_ext/numeric_spec.rb
deleted file mode 100644
index 78d7ec7..0000000
--- a/spec/time_iterator/core_ext/numeric_spec.rb
+++ /dev/null
@@ -1,13 +0,0 @@
-RSpec.describe Numeric do
-  let(:time) { Time.now }
-
-  describe '#quarter(s)' do
-    [:quarter, :quarters].each do |method|
-      it 'adds 3 months per quarter' do
-        expect( time + 1.send(method) ).to eq time + 3.months
-        expect( time + 3.send(method) ).to eq time + 9.months
-        expect( time + 4.send(method) ).to eq time + 1.year
-      end
-    end
-  end
-end
diff --git a/spec/time_iterator/core_ext/time_spec.rb b/spec/time_iterator/core_ext/time_spec.rb
deleted file mode 100644
index 5914234..0000000
--- a/spec/time_iterator/core_ext/time_spec.rb
+++ /dev/null
@@ -1,102 +0,0 @@
-RSpec.describe Time do
-  let(:time) { described_class.now }
-
-  describe '#beginning_of' do
-    periods2methods = {
-      minute: :beginning_of_minute,
-      hour: :beginning_of_hour,
-      day: :beginning_of_day,
-      week: :beginning_of_week,
-      month: :beginning_of_month,
-      quarter: :beginning_of_quarter,
-      year: :beginning_of_year
-    }.freeze
-
-    periods2methods.each do |period, method|
-      it "handles #{period}" do
-        expect(time.beginning_of(period)).to eq time.send(method)
-      end
-    end
-
-    it 'raises on invalid period' do
-      expect { time.beginning_of(:time) }.to raise_error(ArgumentError)
-    end
-  end
-
-  describe '#end_of' do
-    periods2methods = {
-      minute: :end_of_minute,
-      hour: :end_of_hour,
-      day: :end_of_day,
-      week: :end_of_week,
-      month: :end_of_month,
-      quarter: :end_of_quarter,
-      year: :end_of_year
-    }.freeze
-
-    periods2methods.each do |period, method|
-      it "handles #{period}" do
-        expect(time.end_of(period)).to eq time.send(method)
-      end
-    end
-
-    it 'raises on invalid period' do
-      expect { time.end_of(:universe) }.to raise_error(ArgumentError)
-    end
-  end
-
-  describe '#days_in_month/year' do
-    # Make sure it's accounting for a leap year
-    let(:time) do
-      described_class.now.change(year: 2020, month: 2, day: 5)
-    end
-
-    it 'gives the days in the current month' do
-      expect( time.days_in_month ).to eq 29
-      expect( time.days_in_year ).to eq 366
-    end
-  end
-
-  describe '#span_to_end_of_today', travel_to: described_class.now do
-    let(:span_begin) { 7.days.ago }
-    let(:span_end) { described_class.now.tomorrow.beginning_of_day }
-    let(:span) { span_begin.span_to_end_of_today }
-
-    it 'creates an excluding span to the end of today' do
-      expect( span.begin ).to eq span_begin
-      expect( span.end ).to eq span_end
-      expect( span ).to be_exclude_end
-    end
-
-    it 'covers from the start and excluding the end' do
-      # See https://github.com/rubocop-hq/rubocop-rspec/issues/926 for why it isn't using to include.
-      # See https://github.com/rubocop/rubocop-rspec/issues/466 for why its disabled (be_cover(start_span))
-      # rubocop:disable RSpec/PredicateMatcher
-      expect( span.cover?(span_begin) ).to be_truthy
-      expect( span.cover?(span_end) ).to be_falsey
-      expect( span.cover?(span_end - 1.second) ).to be_truthy
-      # rubocop:enable RSpec/PredicateMatcher
-    end
-  end
-
-  describe '#dow' do
-    it 'returns the abbreviated day of the week' do
-      wednesday = described_class.local(2019, 9, 18)
-      expect( wednesday.dow ).to eq 'Wed'
-    end
-  end
-
-  describe '#iso_date' do
-    it 'returns an ISO date' do
-      time = described_class.local(2019, 9, 8)
-      expect( time.iso_date ).to eq '2019-09-08'
-    end
-  end
-
-  describe '#human_date' do
-    it 'formats the date for humans' do
-      sunday = described_class.local(2019, 9, 8)
-      expect( sunday.human_date ).to eq 'Sun Sep 8 2019'
-    end
-  end
-end
diff --git a/spec/time_iterator_spec.rb b/spec/time_iterator_spec.rb
index 3b7d808..e517da9 100644
--- a/spec/time_iterator_spec.rb
+++ b/spec/time_iterator_spec.rb
@@ -1,37 +1,64 @@
 RSpec.describe TimeIterator do
-  let(:time) { Time.now }
+  let(:time) { Time.gm(2000, 1, 1, 0, 0, 0) }
 
   describe '.iterate' do
     it 'uses an Enumerator' do
-      expect( described_class.iterate(time, by: :day) ).to be_a(Enumerator)
+      expect(
+        described_class.iterate(time, by: ActiveSupport::Duration.days(1))
+      ).to be_a(Enumerator)
     end
 
-    it 'raises on an invalid period' do
-      expect do
+    it 'raises on an invalid `by`' do
+      expect {
         described_class.iterate(time, by: :colors)
-      end.to raise_error(ArgumentError)
+      }.to raise_error(ArgumentError)
     end
 
-    [
-      :second, :minute, :hour, :day, :week, :month, :quarter, :year,
-      :seconds, :minutes, :hours, :days, :weeks, :months, :quarter, :years
-    ].each do |period|
-      it "iterates by #{period}" do
-        expect(
-          described_class.iterate(time, by: period).take(3)
-        ).to eq [time, time + 1.send(period), time + 2.send(period)]
-      end
+    it "iterates" do
+      expect(
+        described_class.iterate(
+          Time.gm(2000, 1, 1), by: ActiveSupport::Duration.days(3)
+        ).take(3)
+      ).to eq [
+        Time.gm(2000, 1, 1),
+        Time.gm(2000, 1, 4),
+        Time.gm(2000, 1, 7)
+      ]
+    end
+
+    it "stops before `to`" do
+      expect(
+        described_class.iterate(
+          Time.gm(2000, 1, 1),
+          by: ActiveSupport::Duration.days(3),
+          to: Time.gm(2000, 1, 12)
+        ).to_a
+      ).to eq [
+        Time.gm(2000, 1, 1),
+        Time.gm(2000, 1, 4),
+        Time.gm(2000, 1, 7),
+        Time.gm(2000, 1, 10)
+      ]
     end
 
-    it 'iterates every X periods' do
+    it "includes `to`" do
       expect(
-        described_class.iterate(time, by: :day, every: 3).take(3)
-      ).to eq [time, time + 3.days, time + 6.days]
+        described_class.iterate(
+          Time.gm(2000, 1, 1),
+          by: ActiveSupport::Duration.days(3),
+          to: Time.gm(2000, 1, 10)
+        ).to_a
+      ).to eq [
+        Time.gm(2000, 1, 1),
+        Time.gm(2000, 1, 4),
+        Time.gm(2000, 1, 7),
+        Time.gm(2000, 1, 10)
+      ]
     end
 
     it 'does not alter the original time' do
       orig = time.clone
-      described_class.iterate(time, by: :day).take(5)
+      described_class.iterate(time, by: ActiveSupport::Duration.weeks(2)).take(5)
       expect( time ).to eq orig
     end
   end