![Mamba](https://xoops.org/uploads/cavt4afcaa491477b.jpg)
1) in the file /language/YOURLANGUAGE/calendar.php add or replace this line:
define('_CAL_FORMAT',_SHORTDATESTRING);
2) in the file /language/YOURLANGUAGE/global.php make sure that you have the correct definition for _SHORTDATESTRING, for example:
define('_SHORTDATESTRING', Y-m-d');
Sometimes the local language files from older XOOPS versions miss the new additions like the _SHORTDATESTRING. So make sure that you also update the local language files.