I was able to get the date conversion working on a multiple update page where I had two fields, "date" and "name" by changing the following line of code:
$WA_fieldValuesStr = "".WA_AB_getLoopedFieldValue("textfield", $WA_multipleInsertCounter) ."" . "|" . "".WA_AB_getLoopedFieldValue("textfield2", $WA_multipleInsertCounter) ."";
to:
$WA_fieldValuesStr = "".date("Y-m-d", strtotime(WA_AB_getLoopedFieldValue("date", $WA_multipleInsertCounter))) ."" . "|" . "".WA_AB_getLoopedFieldValue("name", $WA_multipleInsertCounter) ."";