diff --git a/Gemfile b/Gemfile
index ce9565beb..6a5c89321 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 
 # core
-gem 'iso8601',      '0.8.6' # for dates and times
+gem 'iso8601',      '0.12.1' # for dates and times
 gem 'rails', '~> 5.1.7'
 gem 'rest-client'
 gem 'uglifier'
diff --git a/Gemfile.lock b/Gemfile.lock
index 75317c030..b76bfefd0 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -214,7 +214,7 @@ GEM
       concurrent-ruby (~> 1.0)
     i18n_data (0.8.0)
     isikukood (0.1.2)
-    iso8601 (0.8.6)
+    iso8601 (0.12.1)
     jquery-rails (4.3.5)
       rails-dom-testing (>= 1, < 3)
       railties (>= 4.2.0)
@@ -473,7 +473,7 @@ DEPENDENCIES
   grape
   haml (~> 5.0)
   isikukood
-  iso8601 (= 0.8.6)
+  iso8601 (= 0.12.1)
   jquery-rails
   jquery-ui-rails (= 5.0.5)
   kaminari
diff --git a/app/models/concerns/contact/transferable.rb b/app/models/concerns/contact/transferable.rb
index 3f151251a..987933a8d 100644
--- a/app/models/concerns/contact/transferable.rb
+++ b/app/models/concerns/contact/transferable.rb
@@ -3,7 +3,7 @@ module Concerns::Contact::Transferable
 
   included do
     validates :auth_info, presence: true
-    after_initialize :generate_auth_info, if: 'new_record? && auth_info.blank?'
+    after_initialize :generate_auth_info, if: -> { new_record? && auth_info.blank? }
   end
 
   def transfer(new_registrar)
diff --git a/config/initializers/new_framework_defaults.rb b/config/initializers/new_framework_defaults.rb
index 99b2db3b0..2f9c3c5cb 100644
--- a/config/initializers/new_framework_defaults.rb
+++ b/config/initializers/new_framework_defaults.rb
@@ -6,7 +6,8 @@
 #
 # Read the Guide for Upgrading Ruby on Rails for more info on each option.
 
-Rails.application.config.action_controller.raise_on_unfiltered_parameters = true
+# DEPRECATION WARNING: raise_on_unfiltered_parameters is deprecated and has no effect in Rails 5.1.
+#Rails.application.config.action_controller.raise_on_unfiltered_parameters = true
 
 # Enable per-form CSRF tokens. Previous versions had false.
 Rails.application.config.action_controller.per_form_csrf_tokens = true
@@ -19,7 +20,9 @@ Rails.application.config.action_controller.forgery_protection_origin_check = tru
 ActiveSupport.to_time_preserves_timezone = true
 
 # Do not halt callback chains when a callback returns false. Previous versions had true.
-ActiveSupport.halt_callback_chains_on_return_false = false
+# DEPRECATION WARNING: ActiveSupport.halt_callback_chains_on_return_false= is deprecated and
+# will be removed in Rails 5.2.
+#ActiveSupport.halt_callback_chains_on_return_false = false
 
 # Configure SSL options to enable HSTS with subdomains. Previous versions had false.
 Rails.application.config.ssl_options = { hsts: { subdomains: true } }