How to dinamically set a widget attribute in PHP

Summary:

I need to set the max_year attribute from the DateInput widget dynamically

Content (required):

I am trying to do something like this:

<?php $max =  date("Y", strtotime("2 years")); ?>

<rn:widget path="input/DateInput" name="Incident.CustomFields.Patient.Patient_DOB" max_year=<?= $max; ?> label_input="Patients Date of Birth (mm/dd/yyyy):" required="true" />

No errors are displayed, but the rendered HTML ignores the attribute as is

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):

<?php $max =  date("Y", strtotime("2 years")); ?>

<rn:widget path="input/DateInput" name="Incident.CustomFields.Patient.Patient_DOB" max_year=<?= $max; ?> label_input="Patients Date of Birth (mm/dd/yyyy):" required="true" />