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

<?php

function 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:
0
0
-1
1

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");
?>

I typically ignore the comments section on php.net; however, I should note that it is wrong to even attempt comparison between date intervals by adding them to a zero date because PHP's date intervals are stored in ambiguous units of time (days, months, and years are not regular).

a correct date interval is stored in terms of seconds (plus integer microseconds, typically) between the two dates. any more coarse measurement for approximate purposes is then derived from the interval. 

just skip this entirely, and write your own interval type based on the above, from first principals. it's easy.

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