$fields['timestamp'] = BaseFieldDefinition::create('created')
->setLabel(new TranslatableMarkup('Created On'))
->setDescription(t('The time that the node was created.'))
->setDisplayOptions('view', [
'weight' => -2,
'type' => 'timestamp',
'settings' => [
'date_format' => 'custom',
'custom_date_format' => 'd/m/Y H:i:s',
]
])
->setDisplayConfigurable('view', TRUE);