twentythirteen custom date format

dear all,

i have a problem with customization of the date format.

in the admin panel i changed the date format to d/m/Y.

however, my posts still show the default date format. see screenshot: here

if i echo out the date format to check if wordpress is aware of my custom date format via
echo get_option( 'date_format' );
it outputs the correct format, i.e., d/m/Y.

however, the output date is still in the original format as seen in the above screenshot.
what can be the reason?

my config:
i use wordpress 4.1.1 with twentythirteen theme + child theme.