Redmine

At my company we are going to start using Redmine, but we want to load a previous historical issues to keep useful information. After trying to install a plugin import unsuccessfully all I've done is loaded directly into the issues table some ISSUES with phpmyadmin some test issues. Now, from Redmine, I can see the list of all loaded issues but when i go to see one of these issues manually loaded I get an error. The error that occurs is:

Started GET "/redmine/issues/16" for 127.0.0.1 at 2014-01-17 08:57:44 +0100
Processing by IssuesController#show as HTML
Parameters: {"id"=>"16"}
Current user: user (id=1)
Rendered issues/_action_menu.html.erb (78.1ms)
Rendered issues/show.html.erb within layouts/base (343.8ms)
Completed 500 Internal Server Error in 3484ms

ActionView::Template::Error (undefined method `year' for nil:NilClass):
25: <%= render_issue_subject_with_tree(@issue) >
26: </div>
27: <p class="author">
28: <= authoring @issue.created_on, @issue.author >.
29: < if @issue.created_on != @issue.updated_on >
30: <= l(:label_updated_time, time_tag(@issue.updated_on)).html_safe >.
31: < end %>
app/helpers/application_helper.rb:379:in `time_tag'
app/helpers/application_helper.rb:375:in `authoring'
app/views/issues/show.html.erb:28:in `_app_views_issues_show_html_erb___871564060_22613880'
app/controllers/issues_controller.rb:119:in `block (2 levels) in show'
app/controllers/issues_controller.rb:116:in `show'

Can anyone help me out please? thank you very much