diff --git a/lib/time_difference.rb b/lib/time_difference.rb
index 589e202..de9fe89 100644
--- a/lib/time_difference.rb
+++ b/lib/time_difference.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'rubygems'
 require "active_support/all"
 
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 06b4c1d..c207f71 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'rspec'
 require 'time_difference'
 
diff --git a/spec/time_difference_spec.rb b/spec/time_difference_spec.rb
index e1e7f8c..deff6dc 100644
--- a/spec/time_difference_spec.rb
+++ b/spec/time_difference_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe TimeDifference do