Hi.
You need to add something like this in a code pack BeforeStore field :
$date = $fields['date']->value;
$createDate = new DateTime($date);
$onlydate = $createDate->format('Y-m-d');
$fields['date']->value = $onlydate;
[you may need to store the new value permanently in the same or another field? eg. date2]
$name = $fields['date']->storage_field;
$table = $fields['date']->storage_table;
$config['storages'][$table][$name] = $fields['date']->value;