It's the case when $value holds a valid DateInterval as string, that is P1M for example. It's not parsed by `strtotime`. It's not detectable in any other way than using it to create a DateInterval object and catching an exception.

~~~php
$value = $this->normalizeDatetimeValue($value);
if ($value === false) {
   try {
       $interval = new \DateInterval($value);
   } catch (Exception $e) {
       // invalid date/time and invalid interval
       return $this->nullDisplay;
   }
} else {
    $timezone = new \DateTimeZone($this->timeZone);
    $dateNow = new DateTime('now', $timezone);
    $dateThen = new DateTime(null, $timezone);
    $dateThen->setTimestamp($value);

    $interval = $dateNow->diff($dateThen);
}
~~~