Cannot format date for date field in a Views_Database_Connector view
Took me a while but I figured out what I needed to do - create a custom module with a plugin to override the Date.php plugin (which actually overrides the FieldPluginBase.php).  Using the LastTimestamp.php plugin (which overrides the Date.php) from drupal's core comment module as a reference, I was able to piece together what I needed to do.  

First step is when reading in the data from the tables (via the Views_Database_Connector module), I gave the drupal date field a different id rather than the default 'date' (eg "date_to_display").  This is the id used to trigger the call to the custom plugin rather than the Date.php plugin.  While the VDC was the first step, actually giving the date field a different id was the final piece of the puzzle to make this work.

Next step was to create the custom module and then create the plugin to override Date.php and use the id in the annotation (this is important):  @ViewsField("date_to_display").  You can read about this in Drupal's doc Annotations-Based Plugins in Views.  In the custom plugin, override the render function.  In my particular case, I simply just checked to see if the date field value was numeric (which seems to be required for this to not throw an exception), and if not, convert it to timestamp (new value = strtotime(value)).  For me that did the trick. The top part of the created plugin looks like this:

&lt;?php

namespace Drupal\my_module\Plugin\views\field;

use Drupal\views\Plugin\views\field\Date;
use Drupal\views\ResultRow;

/**
 * A handler to provide proper displays for dates.
 *
 * @ingroup views_field_handlers
 *
 * @ViewsField("date_to_display")
 */
    class ViewsDateField extends Date {

  /**
   * {@inheritdoc}
   */
  public function render(ResultRow $values) {