Authored on calendar view throwing errors

Found by audriusb in #2492011: Port Calendar module to Drupal 8 →

Any ideas why I need to change

$item_start_date = \DateTime::createFromFormat($storage_format, $row->{$info['query_name']});
$item_end_date = \DateTime::createFromFormat($storage_format, $row->{$info['query_name']});
in /modules/contrib/calendar/src/Plugin/views/row/Calendar.php

to

$item_start_date = \DateTime::createFromFormat('U', $row->{$info['query_name']});
$item_end_date = \DateTime::createFromFormat('U', $row->{$info['query_name']});
when working with "authored on" date field? field stores timestamp format and seems like I can't use anything else other than 'U' format ... How can I change $storage_format format?