Simple example why you should make an interface over DateTime

Clock.php

<?php

declare(strict_types=1);

interface Clock
{
    public function now(): \DateTimeInterface;
}

DoIt.php

<?php

declare(strict_types=1);

class DoIt
{
    private $clock;
    private $deps;
  
    public function __construct(Clock $clock, SomeDependency $deps)
    {
        $this->clock = $clock;
        $this->deps = $deps;
    }
  
    public function doSomething(): void
    {
       if ($this->clock->now() > (new \DateTimeImmutable('2018-11-01 01:00:00'))) {
          $this->deps->run(); 
       }
    }
}

DoItSpec.php

<?php

declare(strict_types=1);

use PhpSpec\ObjectBehavior;

class DoItSpec extends ObjectBehavior
{
    private $now;

    public function let(Clock $clock, SomeDependency $deps)
    {
        $this->beConstructedWith($clock, $deps);
    }
  
    public function it_do_something_depends_on_time(Clock $clock, SomeDependency $deps)
    {
        $clock->now()->willReturn((new \DateTimeImmutable('2020-12-01 01:00:00')));
        $deps->run()->shouldBeCalled();
      
        $this->doSomething();
    }
}

MockableClock.php

<?php

declare(strict_types=1);

final class MockableClock implements Clock
{
    private $currentTime = null;
    private $clock;
  
    public function __construct(Clock $clock)
    {
        $this->clock = $clock;
    }
  
    public function now(): \DateTimeInterface
    {
        if ($this->currentTime) {
            return $this->currentTime;
        }
        return $this->clock->now();
    }
  
    public function set(\DateTimeInterface $time): void
    {
        $this->currentTime = $time;
    }
  
    public function reset(): void
    {
        $this->currentTime = null;
    }
}

Something.php

<?php

declare(strict_types=1);

final class Something
{
    private $clock;
    public function __construct(Clock $clock)
    {
        $this->clock = $clock;
    }
  
    /**
     * Good way. It easily testable
     */
    public function beforeMidday(): bool
    {   
        return (int) $this->format('Hi') < 1200);
    }
    
    /**
     * This method cannot be tested. It depends on time when test is run.
     */
    public function afterMidday(): bool
    {   
        return (int) (new \DateTimeImmutable())->format('Hi') > 1200);
    }
}

SomethingTest.php

<?php

declare(strict_types=1);

class SomethingTest
{
    /**
     * @dataProvider getTestCases
     * @test
     */
    public function checks_that_current_time_is_before_midday($expected, $currentTime)
    {
        $systemClock = new SystemClock();
        $clock = new MockableClock($systemClock);
      
        $something = new Something($clock);
        $something->set($currentTime);
      
        $this->assertEquals($something->beforeMidday(), $expected);
    }
  
    public function getTestCases()
    {
        return [
            [true, (new \DateTimeImmutable('2018-11-01 01:00:00'))],
            [true, (new \DateTimeImmutable('2018-11-01 11:59:59'))],
            [false, (new \DateTimeImmutable('2018-11-01 12:00:00'))],
            [false, (new \DateTimeImmutable('2018-11-01 22:22:22'))],
        ]
    }
}

SystemClock.php

<?php

declare(strict_types=1);

final class SystemClock implements Clock
{
    public function now(): \DateTimeInterface
    {
        return new \DateTimeImmutable();
    }
}