It seems there is bug in showday when you use a diffenrent language. If you click the event for a day the date in the result is set to 01/01/1970.

Showday works correct if you use english language.

Ahh, it looks like I forgot that strtotime only works for english date strings, and I'm using the month name. This should fix it for you.

Line 177 of showday.inc.php has this:

$date_string = "{$ubbt_lang[$mname]} $day $year 12:01 PM";

Change that, to this:

$date_string = "$month/$day/$year 12:01 PM";

yes, it work