https://github.com/XoopsModules25x/extcal/issues/31
as it can be a source of error!
And you really have to communicate for European date format, you have to use the "-" sign in global.php!
Note:
Dates in the m/d/y or d-m-y formats are disambiguated by looking at the separator between the various components: if the separator is a slash (/), then the American m/d/y is assumed; whereas if the separator is a dash (-) or a dot (.), then the European d-m-y format is assumed. If, however, the year is given in a two digit format and the separator is a dash (-), the date string is parsed as y-m-d.
To avoid potential ambiguity, it's best to use ISO 8601 (YYYY-MM-DD) dates or DateTime::createFromFormat() when possible.
xoops_load('XoopsFormRendererBootstrap4');
XoopsFormRenderer::getInstance()->set(new XoopsFormRendererBootstrap4());
define('_DATESTRING', 'd-m-Y H:i:s');
define('_MEDIUMDATESTRING', 'd-m-Y H:i');
define('_SHORTDATESTRING', 'd-m-Y');
define('_DATESTRING', 'd/m/Y H:i:s');
define('_MEDIUMDATESTRING', 'd/m/Y H:i');
define('_SHORTDATESTRING', 'd/m/Y');