gave the field the modifier '-xslim'

app/assets/stylesheets/costs/costs_legacy.css

@@ -97,9 +97,6 @@ td.comment input {
   cursor: pointer;
 }
 
-#cost_object_fixed_date {
-  width: 10%;
-}
 
 table.progress td.exceeded { background: #E1B9B9 none repeat scroll 0%; }
 

app/views/cost_objects/_form.html.erb

@@ -27,7 +27,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
   <%= f.text_area :description, rows: (@cost_object.description.blank? ? 10 : [[10, @cost_object.description.length / 50].max, 100].min), cols: 60 %>
 </div>
 <div class="form--field">
-  <%= f.text_field :fixed_date %>
+  <%= f.text_field :fixed_date, container_class: '-xslim' %>
   <%= calendar_for :cost_object_fixed_date %>
 </div>