Hi,
There is a little mistake in the algorithm which change the month in text to his number.
Please update the file "administrator/com_hikashop/classes/field.php" and replace the line
$conversion .= 'for(var j=0;j<12;++j){if(Calendar._MN[j].substr(0,elems['.$mP.'].length).toLowerCase()==elems['.$mP.'].toLowerCase()){elems['.$mP.']=j;break;}};
By
$conversion .= 'for(var j=0;j<12;++j){if(Calendar._MN[j].substr(0,elems['.$mP.'].length).toLowerCase()==elems['.$mP.'].toLowerCase()){elems['.$mP.']=(j+1);break;}};
It will fix your problem (and this fix will be include in the next HikaShop release).
Regards,