diff --git a/activesupport/lib/active_support/file_update_checker.rb b/activesupport/lib/active_support/file_update_checker.rb
index 43e8b7c5282f15381999b2204c625c2ae276702b..b5667b6ac87483f967dc5e2d9f7d5274197f0081 100644
--- a/activesupport/lib/active_support/file_update_checker.rb
+++ b/activesupport/lib/active_support/file_update_checker.rb
@@ -1,3 +1,5 @@
+require 'active_support/core_ext/time/calculations'
+
 module ActiveSupport
   # FileUpdateChecker specifies the API used by Rails to watch files
   # and control reloading. The API depends on four methods: