Add basic Period class

spec/period_spec.rb

@@ -0,0 +1,18 @@
+require "spec_helper"
+
+describe Periodicity::Period do
+  around(:each) do |example|
+    Timecop.freeze do
+      example.run
+    end
+  end
+
+  describe "#next" do
+    context "in daily basis" do
+      it "should return the time 2 days from now" do
+        @period = Periodicity::Period.new(2.day)
+        expect(@period.next).to be_eql(2.day.from_now)
+      end
+    end
+  end
+end