private static function buildDatetime(DateTime $base, $amount, DateInterval $interval, $byAddition = true)
	{
		if (intval($amount) < 2)
		{
			throw new \InvalidArgumentException('Amount have to be greater than 2');
		}

		// Start from 2, because start date and end date also count
		for ($i = 2; $i <= $amount; $i++)
		{
			$base = $byAddition ? $base->add($interval) : $base->sub($interval);
		}

		return $base;
	}