You can do this from the twig template:
{{ form_widget(form.birthdate, { 'attr': {'class': 'calendar'} }) }}
From http://symfony.com/doc/current/book/forms.html#rendering-each-field-by-hand
You can do this from the twig template:
{{ form_widget(form.birthdate, { 'attr': {'class': 'calendar'} }) }}
From http://symfony.com/doc/current/book/forms.html#rendering-each-field-by-hand