firefly-iii/resources/lang/en_US/config.php

<?php
declare(strict_types=1);
return [
    'locale'          => 'en, English, en_US, en_US.utf8, en_US.UTF-8',
    'month'           => '%B %Y',
    'month_and_day'   => '%B %e, %Y',
    'date_time'       => '%B %e, %Y, @ %T',
    'specific_day'    => '%e %B %Y',
    'week_in_year'    => 'Week %W, %Y',
    'quarter_of_year' => '%B %Y',
    'year'            => '%Y',
    'half_year'       => '%B %Y',

];