time-sensitive-classes.php

<?php

interface TimeProvider
{
    public function now(): \DateTimeImmutable;
}

class SystemTime implements TimeProvider
{
    public function now(): \DateTimeImmutable
    {
        return new \DateTimeImmutable();
    }
}

class FakeTime implements TimeProvider
{
    public function __construct(
        private \DateTimeImmutable $nowDate
    ){}

    public function now(): \DateTimeImmutable
    {
        return $this->nowDate;
    }
}

class HappyFridayVoucher
{
    public function __construct(
        private ?TimeProvider $timeProvider = null
    ){
	    $this->timeProvider = $timeProvider ?? new SystemTime();
    }

    public function valid(): bool
    {
        return $this->timeProvider->now()->format('l') === 'Friday';
    }
}

$voucher = new HappyFridayVoucher(
    new FakeTime(
        new \DateTimeImmutable('friday')
    )
);

$voucher->valid(); // true