La clase DateInterval

If you want to reverse a date interval use array_reverse and iterator_to_array. I've found using invert to be unreliable.<?php$start_date = date_create("2021-01-01");$end_date   = date_create("2021-01-05"); // If you want to include this date, add 1 day$interval = DateInterval::createFromDateString('1 day');$daterange = new DatePeriod($start_date, $interval ,$end_date);function show_dates ($dr) {    foreach($dr as $date1){        echo $date1->format('Y-m-d').'<br>';    }} show_dates ($daterange);   echo '<br>';// reverse the array$daterange = array_reverse(iterator_to_array($daterange));show_dates ($daterange);           ?>Gives  2021-01-01 2021-01-02 2021-01-03 2021-01-04 2021-01-04 2021-01-03 2021-01-02 2021-01-01

There is a handy way to compare intervals by adding them to 0 dates and comparing dates instead<?phpfunction compare(DateInterval $first, DateInterval $second): int{    $firstDate = (new DateTime())->setTimestamp(0)->add($first);    $secondDate = (new DateTime())->setTimestamp(0)->add($second);    return $firstDate <=> $secondDate;}echo compare(new DateInterval('P2D'), new DateInterval('PT48H')) . PHP_EOL;echo compare(DateInterval::createFromDateString('2 days'), DateInterval::createFromDateString('48 hours')) . PHP_EOL;echo compare(DateInterval::createFromDateString('2 days'), DateInterval::createFromDateString('49 hours')) . PHP_EOL;echo compare(DateInterval::createFromDateString('2 days'), DateInterval::createFromDateString('47 hours')) . PHP_EOL;?>Outputs:00-11

More simple example i use to add or subtract.<?php$Datetime = new Datetime('NOW', new DateTimeZone('America/Bahia'));$Datetime->add(DateInterval::createFromDateString('2 day'));echo $Datetime->format("Y-m-d H:i:s");?>

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 2025-02-01T00:00:00echo "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 $d3->format("Y-m-d\TH:i:s");?>Output: interval total days: 66 daysinterval object years: 0 yearsinterval object months: 2 monthsinterval object days: 5 dayscreate 3 dates that are consecutive, but in two months:$d1 = 2025-01-31T00:00:00$d2 = 2025-02-01T00:00:00$d3 = 2025-02-02T00:00:00Add (negative) interval object to each date. Non-consecutive results:2024-11-26T00:00:002024-11-26T00:00:002024-11-27T00:00:00Use same dates, but subtract total number of days instead of adding interval object. Results (consecutive):2024-11-26T00:00:002024-11-27T00:00:002024-11-28T00:00:00Do month separately (interesting):Subtract 2 months (might not expect this):Jan 31 - 2 months (61 days = Dec, 31 days + Nov, 30 days) ==> Dec 1Feb 1 - 2 months (62 days = Dec, 31 days + Jan, 31 days) ==> Dec 12024-12-01T00:00:002024-12-01T00:00:002024-12-02T00:00:00