Corrected with rubocop cop 'Style/ModuleFunction'.

.rubocop.yml

@@ -212,5 +212,3 @@ Lint/UnreachableCode:
   Enabled: false
 Lint/ShadowingOuterLocalVariable:
   Enabled: false
-Style/ModuleFunction:
-  Enabled: false

app/models/ticket/number/date.rb

@@ -1,7 +1,7 @@
 # Copyright (C) 2012-2014 Zammad Foundation, http://zammad-foundation.org/
 # rubocop:disable ClassAndModuleChildren
 module Ticket::Number::Date
-  extend self
+  module_function
 
   def generate
 

app/models/ticket/number/increment.rb

@@ -1,7 +1,7 @@
 # Copyright (C) 2012-2014 Zammad Foundation, http://zammad-foundation.org/
 
 module Ticket::Number::Increment
-  extend self
+  module_function
 
   def generate