Railss I18n and time-date

I have the following in a simple_form:

<div class='input-group date' id='datetimepickerA'>
          <%= f.text_field (:start), :class => "form-control" %>
          <span class="input-group-addon">
          <span class="glyphicon glyphicon-calendar"></span>
      </span>
        </div>

Which I would like to translate using i18n.l, does anybody know the right synthax please?