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<?php $date = (new \DateTime('2018-07-03'))->modify('+21 days'); echo $date->format('d/m/Y')."\n"; $date = (new \DateTime('2018-07-03'))->add(new \DateInterval('P21D')); echo $date->format('d/m/Y')."\n";
based on UI85A
Output for 7.1.0 - 7.1.25, 7.2.0 - 7.2.33, 7.3.0 - 7.3.26, 7.4.0 - 7.4.14, 8.0.0 - 8.0.2
24/07/2018 24/07/2018