Simple Date helper.

DateTimeHelper.php

<?php
declare(strict_types = 1);

namespace Fapi\Common\DateTime;

use SmartEmailing\Types\Duration;
use SmartEmailing\Types\TimeUnit;

class DateTimeHelpers
{

	public static function diffDays(\DateTimeInterface $first, \DateTimeInterface $second): int
	{
		$first = new \DateTime($first->format('Y-m-d') . ' 00:00:00');
		$second = new \DateTime($second->format('Y-m-d') . ' 00:00:00');
		$diff = $second->diff($first);
		return $diff->days * ($diff->invert ? -1 : 1);
	}

	public static function getDateRangeLength(\DateTimeInterface $begin, \DateTimeInterface $end): int
	{
		$diff = static::diffDays($end, $begin);
		return $diff >= 0 ? $diff + 1 : 0;
	}

	public static function addDays(\DateTimeInterface $date, int $days): \DateTime
	{
		$date = new \DateTime($date->format('Y-m-d H:i:s'));
		return $date->modify($days . ' days');
	}

	public static function addMonths(\DateTimeInterface $date, int $months): \DateTime
	{
		$date = new \DateTime($date->format('Y-m-d H:i:s'));
		return $date->modify($months . ' months');
	}

	public static function setDayOfMonth(\DateTimeInterface $date, int $day): \DateTime
	{
		return static::addDays($date, $day - (int) $date->format('j'));
	}

	public static function isBetweenOrEqual(\DateTimeInterface $date, \DateTimeInterface $beginDate, \DateTimeInterface $endDate): bool
	{
		return self::isGreaterOrEqual($date, $beginDate) &&
			self::isLowerOrEqual($date, $endDate);
	}

	public static function isGreaterOrEqual(\DateTimeInterface $first, \DateTimeInterface $second): bool
	{
		return self::diffDays($first, $second) >= 0;
	}

	public static function isLowerOrEqual(\DateTimeInterface $first, \DateTimeInterface $second): bool
	{
		return self::diffDays($first, $second) <= 0;
	}

	public static function isLower(\DateTimeInterface $first, \DateTimeInterface $second): bool
	{
		return self::diffDays($first, $second) < 0;
	}

	public static function isGreater(\DateTimeInterface $first, \DateTimeInterface $second): bool
	{
		return self::diffDays($first, $second) > 0;
	}

	public static function formatDateTime(\DateTimeInterface $dateTime): string
	{
		return $dateTime->format('Y-m-d H:i:s');
	}

	public static function setDayOfNextMonth(\DateTime $date, int $day): \DateTime
	{
		$month = (int) $date->format('m');
		$year = (int) $date->format('Y');

		$month++;

		if ($month === 13) {
			$year++;
			$month = 1;
		}

		$date->setDate($year, $month, 1);
		return self::setDayOrLastDayOfMonth($date, $day);
	}

	public static function modifyByDurationForPeriodicInvoice(\DateTime $date, Duration $duration, ?int $day): \DateTime
	{
		if ($day && $duration->getUnit()->equalsValue(TimeUnit::MONTHS)) {
			for ($i = 0; $i < $duration->getValue(); $i++) {
				$date = self::setDayOfNextMonth($date, $day);
			}
			return $date;
		}

		return $date->modify($duration->getDateTimeModify());
	}

	public static function setDayOrLastDayOfMonth(\DateTime $date, int $day): \DateTime
	{
		$month = (int) $date->format('m');
		$year = (int) $date->format('Y');
		$lastDayOfMonth = (int) $date->format('t');

		if ($day > $lastDayOfMonth) {
			$day = $lastDayOfMonth;
		}

		$date->setDate($year, $month, $day);

		return $date;
	}

}