Rails - Jul 2006 - render :update do | page | to modify a css class string...

In my booking/_form.rhtml, I have 2 <input text fields>     
startingdate  (selction from unobstrusive calendar ) and endingdate

<div>
<label for="startdate">Booking Start Date</label><br
/>
<%= text_field_tag(''startdate'', @startdate, {:class =>  
@startdateclass, :readonly => "readonly", :maxlength =>
"25"} )%>
<%= observe_field("startdate", :frequency => 0.5,  :url =>  
{:controller => ''booking'' , :action =>
''startdateChanged'' }, :update
=> ''enddate'',
   :with =>
"''input=''+$(''startdate'').value
")%>
</div>

<div>
<label for="enddate">Booking End Date</label><br />
<%= text_field_tag(''enddate'', @enddate, {:class =>  
@enddateclass, :readonly => "readonly", :maxlength =>
"25"} ) %>
</div>

in my controller when a new startingdate is selected, I modify the  
endingdate field value (+10 days) but I should also modify the css  
clas that will toggle some dates in the calendar  ( a format with  
rangelow..)    but I don''t know how to modify the css class string of  
the ending date
the value is correctly set, not the css class when rendering the  
page....

   def startdateChanged
      pd = ParseDate::parsedate(@params["input"])
      tl = Time.local(pd[0], pd[1], pd[2])
      @enddate = (tl+(84600*10)).strftime("%d/%m/%Y")
      render :update do | page |
        page[''enddate''].value = @enddate
        @enddateclass = "format-d-m-y range-low-" + (tl 
+(84600*10)).strftime("%d-%m-%Y")
      end
   end

thanks for your tricks

kad