Example of how to use the date formats set within your sites general settings in the CSV report columns.

example.php

<?php //Please do not add the PHP tag if you already have one.

function tw_reg_date_format( $reg_csv_array, $reg_row ) {

	//Set the columns you can't to convert the date on here.
	$date_columns = array(
		'Ticketdatum',
		'Time registration occurred[REG_date]'
	);

	//Loop through the columns set and use localized date formats.
	foreach($date_columns as $column) {

		//Hardcode the date format?
		//$date_format		= 'D., d.m.y';
		
		//Use the formats set in general settings.
		$date_format 		= get_option( 'date_format' );
		$time_format		= get_option( 'time_format' );

		$reg_csv_array[$column] = date_i18n(  $date_format . ' ' . $time_format, strtotime( $reg_csv_array[$column] ) );
	}

	return $reg_csv_array;
}
add_filter('FHEE__EE_Export__report_registrations__reg_csv_array', 'tw_reg_date_format', 10, 2 );

Ref:

https://eventespresso.com/topic/date-format-of-csv-export-german/