A test gist for https://github.com/CMB2/CMB2/issues/1465

plugin.php

<?php
/**
 * Plugin Name: CMB2 text_datetime_timestamp_timezone Test
 * Test file for CMB2 Bug Report: https://github.com/CMB2/CMB2/issues/1465
 */

add_action(
	'cmb2_admin_init',
	function() {

		$details = new_cmb2_box(
			array(
				'id'           => 'cmb2_test',
				'title'        => __( 'Details', 'cmb2' ),
				'object_types' => array( 'post' ),
			)
		);

		$details->add_field(
			array(
				'name'        => 'Date',
				'type'        => 'text_datetime_timestamp_timezone',
				'id'          => 'date',
				'after_field' => function() {
					$serialized_date = get_post_meta( get_the_ID(), 'date', true );
					if ( ! empty( $serialized_date ) ) {
						$date = maybe_unserialize( $serialized_date );
						echo '<br><br>Date/Time: ' . $date->format( get_option( 'date_format' ) . ' ' . get_option( 'time_format' ) . ' e' ) . '<br>';

					}
				},
			)
		);

	}
);