I can't answer to your mail because it is something with security reason, it doesn't allow me to answer, but I will answer here.
Please go to form->Advanced->More options->Scripts->initialization script->check Custom and put this:
function ff_new_prescrizione_digitale_init()
{
ff_getElementByName('data_prova').value='';
ff_getElementByName('data_consegna').value='';
} // ff_new_prescrizione_digitale_init
new_prescrizione_digitale ->is name of your form (not title)
data_prova -> is name of first calendar (not title)
data_consegna -> is name of second calendar (not title)
