you would need to hand code the recordset loop to use the value from the recordset and use this in a n if statement to trigger the update
something like this:
do {
if ($yourCartName->ConditionalTotal("Quantity", "recID", $row_RecordsetName['recID']) > 0) {
$query = mysql_query('UPDATE books SET purchaseDate = '.$now.' WHERE recID = $row_RecordsetName['recID']);
}
} while ($row_RecordsetName = mysql_fetch_assoc($RecordsetName))