When temporarily overriding a time zone, it is safer to use @Time.use_zone@ than to update Time.zone directly because Time.use_zone will reset the time zone to its original value after the execution of the given block is done.

And RuboCop Rails 2.10.0 will display Rails/TimeZoneAssignment warning when it detects code that directly updates Time.zone.

Index: test/unit/mailer_test.rb
===================================================================
--- test/unit/mailer_test.rb    (リビジョン 20961)
+++ test/unit/mailer_test.rb    (作業コピー)
@@ -386,11 +386,10 @@
     issue = Issue.find(3)
     user = User.find(1)
     %w(UTC Paris Tokyo).each do |zone|
-      Time.zone = zone
-      assert_match /^redmine\.issue-3\.20060719190727\.1@example\.net/, Mailer.token_for(issue, user)
+      Time.use_zone(zone) do
+        assert_match /^redmine\.issue-3\.20060719190727\.1@example\.net/, Mailer.token_for(issue, user)
+      end
     end
-  ensure
-    Time.zone = zone_was
   end
 
   test "#issue_add should notify project members" do