Issue #3348541 by DieterHolvoet: start_time and end_time properties don't work

src/Plugin/Field/FieldType/DateTimeComputed.php

+<?php
+
+namespace Drupal\smart_date\Plugin\Field\FieldType;
+
+use Drupal\Core\Datetime\DrupalDateTime;
+use Drupal\Core\TypedData\DataDefinitionInterface;
+use Drupal\Core\TypedData\TypedDataInterface;
+use Drupal\Core\TypedData\TypedData;
+
+/**
+ * A computed property for dates of Smart Date field items.
+ */
+class DateTimeComputed extends TypedData {
+
+  /**
+   * Cached computed date.
+   *
+   * @var \DateTime|null
+   */
+  protected $date = NULL;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(DataDefinitionInterface $definition, $name = NULL, TypedDataInterface $parent = NULL) {
+    parent::__construct($definition, $name, $parent);
+    if (!$definition->getSetting('date source')) {
+      throw new \InvalidArgumentException("The definition's 'date source' key has to specify the name of the date property to be computed.");
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getValue() {
+    if ($this->date !== NULL) {
+      return $this->date;
+    }
+
+    /** @var \Drupal\Core\Field\FieldItemInterface $item */
+    $item = $this->getParent();
+    $value = $item->{($this->definition->getSetting('date source'))};
+
+    // A date cannot be created from a NULL value.
+    if ($value === NULL) {
+      return NULL;
+    }
+
+    try {
+      $date = DrupalDateTime::createFromTimestamp($value);
+      if ($date instanceof DrupalDateTime && !$date->hasErrors()) {
+        $this->date = $date;
+      }
+    }
+    catch (\Exception $e) {
+      // @todo Handle this.
+    }
+    return $this->date;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setValue($value, $notify = TRUE) {
+    $this->date = $value;
+    // Notify the parent of any changes.
+    if ($notify && isset($this->parent)) {
+      $this->parent->onChange($this->name);
+    }
+  }
+
+}

src/Plugin/Field/FieldType/SmartDateItem.php

@@ -6,7 +6,6 @@ use Drupal\Core\Field\FieldDefinitionInterface;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem;
 use Drupal\Core\TypedData\DataDefinition;
-use Drupal\datetime\DateTimeComputed;
 
 /**
  * Plugin implementation of the 'smartdate' field type.