Hi,
I made the correction on your end directly.
The modification was to edit the view "order / listing" in the backend template and replace:
echo JText::_('FROM').' ';
echo JHTML::_('calendar', hikashop_getDate((@$this->pageInfo->filter->filter_start?@$this->pageInfo->filter->filter_start:''),'%Y-%m-%d %H:%M'), 'filter_start','period_start','%Y-%m-%d',array('size'=>'10','onchange'=>'document.adminForm.submit();'));
echo ' '.JText::_('TO').' ';
echo JHTML::_('calendar', hikashop_getDate((@$this->pageInfo->filter->filter_end?@$this->pageInfo->filter->filter_end:''),'%Y-%m-%d %H:%M'), 'filter_end','period_end','%Y-%m-%d',array('size'=>'10','onchange'=>'document.adminForm.submit();'));
By:
echo JText::_('FROM').' ';
echo JHTML::_('calendar', hikashop_getDate((@$this->pageInfo->filter->filter_start?strtotime(@$this->pageInfo->filter->filter_start):''),'%Y-%m-%d %H:%M'), 'filter_start','period_start','%Y-%m-%d',array('size'=>'10','onchange'=>'document.adminForm.submit();'));
echo ' '.JText::_('TO').' ';
echo JHTML::_('calendar', hikashop_getDate((@$this->pageInfo->filter->filter_end?strtotime(@$this->pageInfo->filter->filter_end):''),'%Y-%m-%d %H:%M'), 'filter_end','period_end','%Y-%m-%d',array('size'=>'10','onchange'=>'document.adminForm.submit();'));