Simple example of getting the last modified date from a child controller

getLastModifiedDate.php

<?php

/**
 * Find the last modified date
 *
 * @return \DateTime
 */
public function getLastModifiedDate()
{
    $modifiedDate = $this->thingRepository->findLastModifiedThing();

    if ($modifiedDate instanceof \DateTime) {
        return $modifiedDate;
    }

    return new \DateTime();
}