Redmine

I solved this problem in my case by overriding Time.now during the import. This ist more like a workaround and maybe it'd be more appropriate to disable the ActiveRecord timestamps at all during import and set created_on and updated_on attributes manually (but since I'm not very familiar with the Redmine models and the acts_as_versioned plugin, I had problems doing so and did this hack to be able to import my project into Redmine without losing the changelog timestamps)

I also added a comment and set the author to nil for the automatic page edit that is done for each completely imported page (where the wiki syntax of Trac is converted to Redmine)

Index: lib/tasks/migrate_from_trac.rake
===================================================================
--- lib/tasks/migrate_from_trac.rake    (revision 1291)
+++ lib/tasks/migrate_from_trac.rake    (working copy)
@@ -69,6 +69,21 @@
                         'developer' => developer_role
                         }

+      class ::Time
+        class << self
+          alias :real_now :now
+          def now
+            real_now - @fake_diff.to_i
+          end
+          def fake (time)
+            @fake_diff = real_now - time
+            res = yield
+            @fake_diff = 0
+           res
+          end
+        end
+      end
+
       class TracComponent < ActiveRecord::Base
         set_table_name :component
       end
@@ -345,14 +360,14 @@
                i.tracker = TRACKER_MAPPING[ticket.ticket_type] || DEFAULT_TRACKER
                i.custom_values << CustomValue.new(:custom_field => custom_field_map['resolution'], :value => ticket.resolution) unless ticket.resolution.blank?
                i.id = ticket.id unless Issue.exists?(ticket.id)
-               next unless i.save
+               next unless Time.fake(ticket.time) { i.save }
                TICKET_MAP[ticket.id] = i.id
                migrated_tickets += 1

                # Owner
             unless ticket.owner.blank?
               i.assigned_to = find_or_create_user(ticket.owner, true)
-              i.save
+              Time.fake(ticket.time) { i.save }
             end

                # Comments and status/resolution changes
@@ -380,7 +395,7 @@
                                                :old_value => resolution_change.oldvalue,
                                                :value => resolution_change.newvalue)
               end
-              n.save unless n.details.empty? && n.notes.blank?
+              Time.fake(time) { n.save } unless n.details.empty? && n.notes.blank?
                end

                # Attachments
@@ -390,7 +405,7 @@
               a.file = attachment
               a.author = find_or_create_user(attachment.author)
               a.container = i
-              migrated_ticket_attachments += 1 if a.save
+              migrated_ticket_attachments += 1 if Time.fake(attachment.time) { a.save }
                end

                # Custom fields
@@ -426,7 +441,7 @@
             p.content.text = page.text
             p.content.author = find_or_create_user(page.author) unless page.author.blank? || page.author == 'trac'
             p.content.comments = page.comment
-            p.new_record? ? p.save : p.content.save
+            Time.fake(page.time) { p.new_record? ? p.save : p.content.save }

             next if p.content.new_record?
             migrated_wiki_edits += 1
@@ -439,13 +454,15 @@
               a.file = attachment
               a.author = find_or_create_user(attachment.author)
               a.container = p
-              migrated_wiki_attachments += 1 if a.save
+              migrated_wiki_attachments += 1 if Time.fake(attachment.time) { a.save }
             end
           end

           wiki.reload
           wiki.pages.each do |page|
             page.content.text = convert_wiki_text(page.content.text)
+            page.content.author = nil
+            page.content.comments = 'Migration to Redmine'
             page.content.save
           end
         end

Committed in r1302 with slight changes (eg. use ticket changetime instead of time since Time.fake is used to properly set updated_on properties).