PSR: should I add basic classes to the namespace section?

Question

Some people add basic classes like Exception, Throwable, DateTime etc to the namespace section like this:

<?php
namespace Foo\Bar;

use DateTime;
use Throwable;

Class MyClass{

    public function getDate()
    {
        try {
            new DateTime('Bizarre value');
        } catch (Throwable $exception) {
            //error handling
        }
    }
}

(new MyClass())->getDate();

But other people just add a slash in front of those basic classes:

<?php
namespace Foo\Bar;

Class MyClass{

    public function getDate()
    {
        try {
            new \DateTime('Bizarre value');
        } catch (\Throwable $exception) {
            //error handling
        }
    }
}

(new MyClass())->getDate();

What is the "canonical" one according to PSR?