(PHP 5 >= 5.5.0, PHP 7)
DateTimeImmutable::add — Adds an amount of days, months, years, hours, minutes and seconds
public DateTimeImmutable::add ( DateInterval $interval ) : DateTimeImmutable
Like DateTime::add() but works with DateTimeImmutable.
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https://www.php.net/manual/en/datetimeimmutable.add.php