Steps to produce this:

prompt>rails test_app
prompt>cd test_app
prompt>script/generate scaffold date_test my_date:datetime
prompt>rake db:migrate

now edit your app/views/date_tests/edit.html.erb:

<h1>Editing date_test</h1>

<% form_for(@date_test) do |f| %>
  <%= f.error_messages %>
  <p>
    RIGHT!<br/>
    <%= text_field_tag @date_test, f.object.my_date %>
  </p>
  <p>
    WRONG!<br />
    <%= f.text_field :my_date %>
  </p>
  <p>
    <%= f.submit 'Update' %>
  </p>
<% end %>

<%= link_to 'Show', @date_test %> |
<%= link_to 'Back', date_tests_path %>

now edit your config/environment.rb:

#add this
config.time_zone = 'Central Time (US & Canada)'

This recreates the problem I am having in my actual app. The problem with my app is that I'm storing a date in a hidden field and rendering a "user friendly" version. Creating a resource works fine but as soon as I try to edit it the time changes (it adds the difference between my current time zone configuration and UTC). go to http://localhost:3000/date_tests/new and save the time then go to reedit it and you will have two different representations of the date/time one which will save incorrectly and the other that will.