problem with typo6.0.8

hi all
i am using typo6.0.8 in rails3…
when i click on articles tab of typo admin page in browser… i get the following error… and all the remaining tags are working fine.

undefined method `gsub!' for 2011-08-21 06:28:15 UTC:Time

Extracted source (around line #34):

31: %></td>
32: <td><%= collection_select_with_current(:search, :user_id, User.find(:all), "id", "name", @search[:user_id].to_i, true) %></td>
33: <td>

34: <%= collection_select_with_current(:search, :published_at, Article.find_by_published_at, "publication", "publication", @search[:published_at], true) %>
35: </td>
36: <td><input type="submit" value='<%= _("Filter") %>' class='ed_button' />