Control Webform's date field (datepicker) from JS
You can override it with js .. the trick is when it is when your js is attached.

so in your module you need ....
&lt;?php
function MYMODULE_form_alter(&amp;$form, FormStateInterface $form_state, $form_id){
  if ($form_id === 'webform_submission_YOURFORMID') {
    $form['#attached']['library'][] = 'YOURMODULE/mymodule.fix-datepicker';
  }
}


then in your js please try some thing like..

(function ($, Drupal, drupalSettings) {
  'use strict';
  Drupal.behaviors.fixMyDatepicker = {
attach: function (context) {

  var dateElement = $('#edit-1'); // id of your date picker 
  var dateData = dateElement.data();
  if (dateData.datepicker) {
    makeChangesToDatePicker();
  }
  else {
    var myInterv = setInterval(function() {
      if (dateData.datepicker) {
        makeChangesToDatePicker();
        clearInterval(myInterv);
      }
    }, 100);
  }

    function makeChangesToDatePicker() {
      //@TODO here check if mobile .
      var settings = dateData.datepicker.settings;
      settings.maxDate = '2019-03-11';
    }
   }
  };
 })(jQuery, Drupal, drupalSettings);


the above worked for me. remember datepicker is just: https://jqueryui.com/datepicker/