How to have the ISO20022 XML print the max of PaymentDueDate or PaymentDate

We are using the ISO20022 UNSTRUCTURED XML format. I want to print the max of PaymentDueDate or the PaymentDate as the payment execution date on the file .

The m If i try and print one of the 2 dates they print property in the XML. This proves that both the Variables have legitimate values.

However when i try to get the max it doesn't work. Can anyone please help?

                                                <ReqdExctnDt>

                                                        <xsl:value-of select="max(PaymentDate,PaymentDueDate)"/>

                                                </ReqdExctnDt>