Hello Mohamed, (we met at the joomladay last week 
I discussed this afternoon with Jerome about this issue, the issue is real, but there is a workaround using the "dynamic calendar" instead of "simple datapicker".
Like this I don't have anymore problem.
P.S: The bug is in classes/field.php
line 2372:
return JHTML::_('calendar', $value, $map,$this->prefix.$field->field_namekey.$this->suffix,$format,$size);
$size is a string but on Joomla 3 should be an array. That is the problem. But with the other calendar it's working. so it's ok for me.