note 130008 added to class.dateinterval

Subtracting months may not work as you might expect. Code shows how subtracting 2 months from consecutive days produces potentially unexpected results. <?php $origin = date_create('2025-01-31T00:00:00'); $target = date_create('2024-11-26T00:00:00'); $interval = date_diff($origin, $target); echo "interval total days: ".$interval->format('%a days'); echo "interval object years: ".$interval->format('%y years'); echo "interval object months: ".$interval->format('%m months'); echo "interval object days: ".$interval->format('%d days'); // create 3 dates that are consecutive, but in two months $d1 = date_create('2025-01-31T00:00:00'); $d2 = date_create('2025-02-01T00:00:00'); $d3 = date_create('2025-02-02T00:00:00'); // add (negative) interval object to each date $d1->add($interval); $d2->add($interval); $d3->add($interval); echo "Add (negative) interval object to each date. Non-consecutive results: "; echo($d1->format("Y-m-d\TH:i:s")); echo($d2->format("Y-m-d\TH:i:s")); echo($d3->format("Y-m-d\TH:i:s")); // reset dates $d1 = date_create('2025-01-31T00:00:00'); $d2 = date_create('2025-02-01T00:00:00'); $d3 = date_create('2025-02-02T00:00:00'); // Use same dates, but subtract total number of days instead of adding interval object: "; $d1->sub(new DateInterval('P'.$interval->format('%a').'D')); $d2->sub(new DateInterval('P'.$interval->format('%a').'D')); $d3->sub(new DateInterval('P'.$interval->format('%a').'D')); echo "Use same dates, but subtract total number of days instead of adding interval object. Results (consecutive): "; echo($d1->format("Y-m-d\TH:i:s")); echo($d2->format("Y-m-d\TH:i:s")); echo($d3->format("Y-m-d\TH:i:s")); $d1 = date_create('2025-01-31T00:00:00'); $d2 = date_create('2025-02-01T00:00:00'); $d3 = date_create('2025-02-02T00:00:00'); // do month separately for 2025-02-01T00:00:00 echo "Do month separately (interesting): "; echo ('Subtract '. $interval->format('%m').' months (might not expect this):'); echo ('Jan 31 - 2 months (61 days = Dec, 31 days + Nov, 30 days) ==> Dec 1'); echo ('Feb 1 - 2 months (62 days = Dec, 31 days + Jan, 31 days) ==> Dec 1'); $d1->sub(new DateInterval('P'.$interval->format('%m').'M')); echo $d1->format("Y-m-d\TH:i:s"); $d2->sub(new DateInterval('P'.$interval->format('%m').'M')); echo $d2->format("Y-m-d\TH:i:s"); $d3->sub(new DateInterval('P'.$interval->format('%m').'M')); echo $d3->format("Y-m-d\TH:i:s"); ?> Output: interval total days: 66 days interval object years: 0 years interval object months: 2 months interval object days: 5 days create 3 dates that are consecutive, but in two months: $d1 = 2025-01-31T00:00:00 $d2 = 2025-02-01T00:00:00 $d3 = 2025-02-02T00:00:00 Add (negative) interval object to each date. Non-consecutive results: 2024-11-26T00:00:00 2024-11-26T00:00:00 2024-11-27T00:00:00 Use same dates, but subtract total number of days instead of adding interval object. Results (consecutive): 2024-11-26T00:00:00 2024-11-27T00:00:00 2024-11-28T00:00:00 Do month separately (interesting): Subtract 2 months (might not expect this): Jan 31 - 2 months (61 days = Dec, 31 days + Nov, 30 days) ==> Dec 1 Feb 1 - 2 months (62 days = Dec, 31 days + Jan, 31 days) ==> Dec 1 2024-12-01T00:00:00 2024-12-01T00:00:00 2024-12-02T00:00:00 ---- Server IP: 45.112.84.4 Probable Submitter: 45.112.84.18 (proxied: 72.160.109.170) ---- Manual Page -- https://php.net/manual/en/class.dateinterval.php Edit -- https://main.php.net/note/edit/130008 Del: integrated -- https://main.php.net/note/delete/130008/integrated Del: useless -- https://main.php.net/note/delete/130008/useless Del: bad code -- https://main.php.net/note/delete/130008/bad+code Del: spam -- https://main.php.net/note/delete/130008/spam Del: non-english -- https://main.php.net/note/delete/130008/non-english Del: in docs -- https://main.php.net/note/delete/130008/in+docs Del: other reasons-- https://main.php.net/note/delete/130008 Reject -- https://main.php.net/note/reject/130008 Search -- https://main.php.net/manage/user-notes.php