[Rails I18n] AJAX link_to_remote or form_remote_tag

In one of my forms, I have 2 date input fields (start_date and  
end_date) each one depending on the other ( need to add/substract  
variable period)

I wrote that , but how the methods start_changed or end_changed can  
modify the date value to add/sbstract a period before the replacement  
of teh corresponding div.. ?

<%= link_to_remote("starting_date", :update => 'dd_enddate', url =>  
{:action => :start_changed} ) %>
        <span id='dd_startdate'>
          <label for="startdate">Booking Start Date</label><br />
          <%= text_field "bookings", "startdate",  options = {:class =>  
'format-d-m-y', :maxlength => "15" } %>
        </span>

<%= link_to_remote("starting_date", :update => 'dd_startdate', url =>  
{:action => :end_changed} ) %>
        <span id='dd_enddate'>
          <label for="enddate">Booking End Date</label><br />
          <%= text_field "bookings", "enddate",  options = {:class =>  
'format-d-m-y', :maxlength => "15" } %>
        </span>

or should I use a form_remote_tag ? but these 2 fields area already  
within a form ... does it matter ?

thanks for your help

joss