diff --git a/activesupport/lib/active_support/core_ext/date_time/calculations.rb b/activesupport/lib/active_support/core_ext/date_time/calculations.rb
index <HASH>..<HASH> 100644
--- a/activesupport/lib/active_support/core_ext/date_time/calculations.rb
+++ b/activesupport/lib/active_support/core_ext/date_time/calculations.rb
@@ -10,7 +10,11 @@ class DateTime
     end
   end
 
-  # Seconds since midnight: DateTime.now.seconds_since_midnight.
+  # Returns the number of seconds since 00:00:00.
+  #
+  #   DateTime.new(2012, 8, 29,  0,  0,  0).seconds_since_midnight # => 0
+  #   DateTime.new(2012, 8, 29, 12, 34, 56).seconds_since_midnight # => 45296
+  #   DateTime.new(2012, 8, 29, 23, 59, 59).seconds_since_midnight # => 86399
   def seconds_since_midnight
     sec + (min * 60) + (hour * 3600)
   end
Better doc for AS::DateTime#seconds_since_midnight

Adds examples and keeps coherent with the documentation of the
similar method `seconds_until_end_of_day`. [ci skip]