Hi Hendrik
Which version of SermonSpeaker and Joomla are you using and where exactly is
this date format?
The date format can be set in the settings of SermonSpeaker, can you check
if it happens with all available options there?
Also the date format is based on the language setting, and thus depends on
the global Joomla language files. Maybe also check if those are the most
recent files.
Greetings
Thomas