Ok, I found the problem. First of all for joomla 2.5 you need to follow the date syntax:
php.net/manual/en/function.date.php
it will be something like d-m-Y and not %d-%m-%Y...
And now we come to the bug.. In components/com_hotspots/views/json/tmpl/default_description.php on line 30 we have:
<?php echo hotspotsUtils::getLocalDate($this->hotspot->postdate);?>
and it should be jsut
<?php echo ($this->hotspot->postdate);?>
It is a bug, because in the view.raw.php file we already have parsed the date with hotspotsUtils::getLocalDate($this->hotspot->postdate) and if we try to parse it again in the default_description, then the jdate function can't handle it...
Make the change it should work.
Cheers,
Daniel