date_add gets Boolean instead of DateTime - php

Question

I have a very strange error in my Symfony app and I don't know how to fix it. I want to set a DateTime in one of my entities with a DateTime after adding a special DateInterval. Here is my code:
$dateInterval = new \DateInterval('P1DT6H');
$snoozedTo = date_add($sickNote->getSickTo(), $dateInterval);
$channelUser->setSnoozedTo($snoozedTo);
$this->em->persist($channelUser);
In my UnitTests I get the following error:
date_add() expects parameter 1 to be DateTime, boolean given
So I double check if $sickNote->getSickTo() really returns a DateTime. A dump() returns:
DateTime {#17791
+"date": "2018-01-01 06:00:00.000000"
+"timezone_type": 3
+"timezone": "UTC"
}
get_class() on the object returns DateTime.
Here is additional my getter method for the field:
/**
* Get sickTo
* #return Date
*/
public function getSickTo() {
return $this->sickTo;
}
Why date_add gets a Boolean and not the DateTime object?
Edit: I also tried the objet method DateTime::add(). Same result.
Edit²: Here is my declaration of sickTo:
/**
* #var datetime $sickTo
* #ORM\Column(type="date", nullable=true)
* #Assert\Date()
*/
protected $sickTo;

Answer 1

The declaration of sickTo is wrong. you declare it as a Date but you use a Datetime. Try to change that with something like that :
/**
* #var \DateTime $sickTo
* #ORM\Column(type="datetime", nullable=true)
* #Assert\DateTime()
*/
protected $sickTo;