diff --git a/railties/guides/source/active_support_core_extensions.textile b/railties/guides/source/active_support_core_extensions.textile
index a3cb864796f487675325a0c56183c5faa42e0525..091a6392704f4b105435a6aa699833b272b3e1e7 100644
--- a/railties/guides/source/active_support_core_extensions.textile
+++ b/railties/guides/source/active_support_core_extensions.textile
@@ -2652,6 +2652,8 @@ h4. Calculations
 
 All the following methods are defined in +active_support/core_ext/date/calculations.rb+.
 
+WARNING: The following calculation methods have edge cases in November 1582, since days 5..14 just do not exist. This guide does not document their behavior around those days for brevity, but it is enough to say that they do what you would expect. That is, +Date.new(1582, 10, 4).tomorrow+ returns +Date.new(1582, 10, 15)+ and so on. Please check +test/core_ext/date_ext_test.rb+ in the Active Support test suite for expected behavior.
+
 h5. +Date.current+
 
 Active Support defines +Date.current+ to be today in the current time zone. That's like +Date.today+, except that it honors +Time.zone_default+. It also defines +Date.yesterday+ and +Date.tomorrow+, and the instance predicates +past?+, +today?+, and +future?+, all of them relative to +Date.current+.