diff --git a/lib/time_iterator.rb b/lib/time_iterator.rb
index 1502e48..5cc3d8d 100644
--- a/lib/time_iterator.rb
+++ b/lib/time_iterator.rb
@@ -18,29 +18,17 @@ module TimeIterator
   }.freeze
   ITERATE_BY = (PERIODS.keys + PERIODS.values).freeze
 
-  private def valid_period?(period)
-    raise ArgumentError, "Unknown time period: #{period}" unless PERIODS.include?(period)
-  end
-
-  private def method_for_period(method, period)
-    period = period.to_sym
-    valid_period?(period)
-    send("#{method}_#{period}")
-  end
+  class << self
+    def iterate(start, by:, every: 1)
+      by = by.to_sym
 
-  def iterate(by:, every: 1)
-    by = by.to_sym
+      raise ArgumentError, "Unknown period to iterate by: #{by}" unless ITERATE_BY.include?(by)
 
-    raise ArgumentError, "Unknown period to iterate by: #{by}" unless ITERATE_BY.include?(by)
-
-    Enumerator.new do |block|
-      (0..INFINITY).each do |num|
-        block << (self + (num.send(by) * every))
+      Enumerator.new do |block|
+        (0..INFINITY).each do |num|
+          block << (start + (num.send(by) * every))
+        end
       end
     end
   end
 end
-
-class Time
-  include TimeIterator
-end
diff --git a/lib/time_iterator/core_ext/time.rb b/lib/time_iterator/core_ext/time.rb
index ddedf73..0d61719 100644
--- a/lib/time_iterator/core_ext/time.rb
+++ b/lib/time_iterator/core_ext/time.rb
@@ -1,5 +1,15 @@
 # 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
diff --git a/spec/time_iterator/core_ext/time_spec.rb b/spec/time_iterator/core_ext/time_spec.rb
index 41a08c2..5914234 100644
--- a/spec/time_iterator/core_ext/time_spec.rb
+++ b/spec/time_iterator/core_ext/time_spec.rb
@@ -1,14 +1,6 @@
 RSpec.describe Time do
   let(:time) { described_class.now }
 
-  describe 'period check' do
-    it 'raises on invalid period' do
-      expect { time.beginning_of(:time) }.to raise_error(ArgumentError)
-      expect { time.end_of(:universe) }.to raise_error(ArgumentError)
-      expect { time.iterate(by: :colors) }.to raise_error(ArgumentError)
-    end
-  end
-
   describe '#beginning_of' do
     periods2methods = {
       minute: :beginning_of_minute,
@@ -25,6 +17,10 @@
         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
@@ -43,6 +39,10 @@
         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
diff --git a/spec/time_iterator_spec.rb b/spec/time_iterator_spec.rb
index fbe3be2..3b7d808 100644
--- a/spec/time_iterator_spec.rb
+++ b/spec/time_iterator_spec.rb
@@ -1,9 +1,15 @@
 RSpec.describe TimeIterator do
   let(:time) { Time.now }
 
-  describe '#iterate' do
+  describe '.iterate' do
     it 'uses an Enumerator' do
-      expect( time.iterate(by: :day) ).to be_a(Enumerator)
+      expect( described_class.iterate(time, by: :day) ).to be_a(Enumerator)
+    end
+
+    it 'raises on an invalid period' do
+      expect do
+        described_class.iterate(time, by: :colors)
+      end.to raise_error(ArgumentError)
     end
 
     [
@@ -12,20 +18,20 @@
     ].each do |period|
       it "iterates by #{period}" do
         expect(
-          time.iterate(by: period).take(3)
+          described_class.iterate(time, by: period).take(3)
         ).to eq [time, time + 1.send(period), time + 2.send(period)]
       end
     end
 
     it 'iterates every X periods' do
       expect(
-        time.iterate(by: :day, every: 3).take(3)
+        described_class.iterate(time, by: :day, every: 3).take(3)
       ).to eq [time, time + 3.days, time + 6.days]
     end
 
     it 'does not alter the original time' do
       orig = time.clone
-      time.iterate(by: :day).take(5)
+      described_class.iterate(time, by: :day).take(5)
       expect( time ).to eq orig
     end
   end