Setting one date depending on an other

/*On change of from_date (=this element) set to_date  2 days after.

termin_reservation___bis = full element name of to_date, form_18 = your form id

*/

var to_date_id = 'termin_reservation___bis';

var to_date = Fabrik.getBlock('form_18').formElements.get(to_date_id);

var from_date = this.get('value');

var newDate = new Date(from_date);

newDate.increment('day', 2);

to_date.set(newDate);